I live not far from Oklahoma City, where is located a wonderful banjo museum. When you visit this wonderful place, you will see a plethora of banjos of all kinds. Most of what you will find, however, is tenor banjos. The tenor or plectrum banjo is associated with "classical" banjo, Dixieland, Swing, Jazz, and Irish music predominantly. It was the dominant type of banjo for the Twentieth Century. There were community banjo orchestras all over America. They played the plectrum (pick) style of banjo. There were mandolin-banjos with eight srings, four-string tenor banjos, cello banjos, and even bass banjos! The five-string banjo had been around since the times of slavery in America, but people who wanted to play popular music needed to play in various keys. The fifth, or drone, string on the five-string banjo is an impediment in that case. If you're playing in A-flat, you don't want to risk playing an un-muted G string on the top. Players just dispensed with that high drone string altogether and found the banjo to be much more versatile.
In the early Twentieth Century, the guitar really hadn't come into its own yet. In particular, the flat-top guitar didn't have the power to play in a band with horns and percussion until archtop guitars were developed. When that happened, many musicians who played in Jazz and Swing bands switched over to the guitar. A cross-pollenization took place during that transition. Some manufacturers made tenor guitars, with four strings, tuned in fifths, like the tenor banjo so banjo players could get a guitar sound. Conversely, some banjo companies produced six-string banjos so guitar players could cover songs that called for banjo.
That's how the instrument came to be. Historically, the banjitar is a member of the banjo family. Bluegrass didn't become a genre of music until Bill Monroe popularized it in the 1940s. Since that time, the five-string banjo has become the instrument most people think of when they hear the word "banjo" but there is much more to the banjo world than the five-string. The snooty bluegrass purist probably doesn't know any of this. That's not a big deal. He probably can't count to twenty-one without having to pull his pants down. (Just kidding! I love the five-string banjo!)
So that's why the instrument exists. The banjitar enjoyed a brief spate of popularity with the early 1960s folk boom and kind of died off. In the early 2010s, a new wave of acoustic music began to emerge and some long-ignored instruments enjoyed a renaissance, particularly ukeleles and banjitars. Keith Urban and Taylor Swift used banjitars in some of their songs, which elevated its visibility. Now you can find them in the catalogs of Musicians Friend and other online retailers because several guitar manufacturers have started to offer them again. This could be a fad for awhile, but it's a good one because there are more choices of models and brands than there has been in a long time. It's a "Banjitar Renaissance."
The next question is "Why should you play one?" There are a lot of reasons to buy and play a banjitar. Maybe you do home recording and having banjo tracks on your songs gives them something special. Maybe you're like me, a person who loves banjo music, but after decades of playing guitar, you just can't get the knack of that high drone string on the five-string banjo. You're just used to having the bass strings under the thumb of your picking hand. Perhaps you feel limited by the tuning of the five-string banjo, which is traditionally tuned to open G. It is a pain in the butt to capo a five-string banjo so playing in other keys is not easy.
A mistake that people make, is assuming that you're trying to sound like a Bluegrass banjo. That's something that the guitar tuning (EADGBE) of the banjitar doesn't do well. You can modify the tuning and play with the various string gauges (as shown in this video) to more closely approximate the Bluegrass sound, but it's worth exploring the banjitar's own inherent properties.
The six-string banjo is its own instrument. It's not "trying" to be anything else. It has a distinct timber and tone. It is a different instrument than the five-string and the tenor banjo. It has an extended range, which can be used to good effect, musically. It has a different touch than its cousin, the six-string guitar. I think this is a common mistake that many guitarists make at the first. They think that, just because the tuning and the chord shapes are the same, they can play the banjitar just as easily. It's not the case. You have to learn to strike the strings differently and there is a different range of tone colors available. They dynamic range of the banjo far exceeds that of the acoustic guitar. It takes practice to make a banjitar sound good, even for an accomplished guitarist.
Perhaps I had an intuitive sense of this because I play 12-string guitar. The same thing applies with that instrument. Just because it is tuned the same and the chord shapes are the same, a person accustomed to playing six-string guitar might be able to make some decent sounds on the instrument, but they really can't begin to take advantage of the unique properties of it. It takes time and practice. Mastering the six-string banjo also takes time and practice. You need to find your voice on the instrument.
The banjitar has a different feel and balance. You don't hold it like a guitar. You hold it in your lap like a banjo. On a strap, it hangs differently. The fretting hand position is slightly different because of the angle and the long neck. Depending on whether or not you have an open-back or closed-back banjitar, the distance of the top of the banjo from your body varies, affecting the position of the picking hand. You may play it plectrum style for Dixieland or Swing or you might play it with fingerpicks. Your approach may change after you spend time with it. I had played guitar with fingerpicks and I thought I would do the same with the banjitar, but I was wrong. I found I liked the sound of my bare fingers on the strings more than using picks. Getting volume out of a banjo is no problem, even when playing softly, so I could play more relaxed, without picks, and get a more pleasing tone. Of course, tone is subjective, but I found that the banjitar could sound almost like a lute, when played with bare fingers.
The banjitar has a lighter touch. You don't have to strike the strings hard to project. The strings don't ring quite as long as on a guitar; therefore, you have to make sure your timing is good. The angle and placement of your picking hand can bring out a much wider variety of tones very easily. If you are an aging guitarist, there is much less string tension so even arthritic fingers might be able to coax some good sounds out of the banjitar.
When you take the banjitar as its own thing, not just as a guitar that sounds like a banjo, it opens up some interesting avenues. You'll play things you never thought to play or you'll play them in a different way. That's when the fun really starts. That's a good place to conclude, because really, it's what the banjitar is all about, having fun. Once you establish yourself and find your voice on the banjitar, it can become a defining musical step for you.