For bass, it gives you more tonal range. It's hard to play if you're not use to it because at slight shift like of half a cm can really change the whole sound. So you have to be accurate in playing. Not just anyhow hantam.
fretless guitars exist, and aplenty i believe. a random search on google should get u a decent list of musicians who wield their fretless guitars with much authority. the music playable with a fretless is unusual, imho. u cant shred on it, u can strum that well (i think) on it also. its best used with a sustainer, then u can really make some neat sounds