I agree with all the above, though I think the answer is that they are easier to clean. Some will pivot out from the tub like a door, and even those that don't can still be wiped down easier. I also think this is the brainchild of a dumb interior decorator who liked the looklin in one particular setting where you had a large open shower and drains everywhere and it just caught on because monkey see monkey do. In my home we ended up hanging a shower curtain on a spring rod to defeat this silliness and not step out into a slippery lake of bath water every day.
Regards,
-Bouncer-