Ok, let's take a look at this from 2 points, a) musically and b) practically.
Music wise, the fretless has a very unique sound, its tone is often described as 'mwah', of course with some quick EQ adjustments, I believe you can get rid of this vocal effect, the lack of frets gives you a beautiful legato effect when you move across the fingerboard, a great advantage of this, is that you can get really nice horizontal vibratos and bend one or more notes a range of less than a quarter-tone to more than a full tone (But that would really just be sliding). Markent17 also brought up a good point that I agree to, that is, the fretless (can) grant you the freedom to be more expressive, the way you slide between notes at different speeds, pull off different bends and utilize legato creates tones and phrases not possible on a fretted.
From a practical viewpoint there isn't as much, you no longer have to worry about fretbuzz and cleaning the fretboard is bit easier as you can access more of it, that's really all I can think of.