Can't speak for anyone else (obviously!!!) but I just kinda picked it up & played it.
'It' was (at the time) an unlined Vintage Thumb copy in a music shop in Melton.
I bought my Precious (Westone Thunder IIIa Mk2 Fretless Unlined) and have been happy ever since.
It's one of those things, I think, you either get it or you don't. Like 5 strings & narrow string spacing.
I like all of the above.
As for tone etc, there isn't the attack of a fretted, but there's more subtlety. You can play straight or with 'Mwha!' If I use my thumb, there's 'Mwha'.
Once you're used to it, it's like being set free.
G.