Jim notes:
.Personally, after years of dealing with old stuff with carriage bolts

in them, I use them only when I have to. If there is any way I can
reasonably use a hex head, I do. A little rust, wear or aging & a
carriage bolt can be a pain to remove for repairs.

True, but if you can't reach the head side of the bolt...no substitute.
Bolt-through ledger boards for decks come to mind. Hard to be both
inside and outside, and often the helper is busy elsewhere. Too, if
there's a chance a hex head will snag on something the round head of
the CB is preferable. Otherwise, I don't see much need--though they are
somewhat prettier than hex heads, I guess, and a lot of makers of
sheetmetal stands use them (which, as a too frequent tool assembler, I
really do appreciate).