First, why do you ask?
Second, I play because I love making music. The simple act of making organized noise over time makes me happy. My emotional well-being - stress, etc - is much better after I've practiced. I settled on UB because while at first I was curious, the more I worked at it, the more I enjoyed the challenge of it and its intrinsic voices, both pizz and arco, and I appreciated how much better it made my musicianship - I find I can't play a note in tune if I can hear it in my mind's ear before I go to finger the note - that makes my playing purposeful and intended rather than spastic noise making - shredding. The difference between a cogent speaker and a nervous teenager during an uncomfortable pause in the conversation. I play guitar pretty well too, but I haven't fully developed my own voice with it yet and I won't focus on it again until after I've mastered the UB. I'm still working on refining my UB voice with more sophisticated note choices and techniques.
Lastly, I love how powerful the UB can be in a band. If I lay down the groove solidly and articulate the harmony the way I hear it, the band should respond to my statement. If I decide that that E7 is going to be a Bm11 on bar 2, the other players best recognize that that's what I've done lest they sound totally un-hip and reveal their lack of musicianship. Similarly, where I place the beat will make good players change their relationship with the beat. This last part is just about bass and could be achieved with a fretted BG, but I love the depth of tone I get from my upright and I no longer play BG at all, mostly because I don't have the bandwidth to try to play two very different instruments well.