One reason noone's given, I think, is that a solo - or an instrumental break, since it's only a solo if it's one musician given a chance to show off, as opposed to ensemble playing - can be a way to allow the missing words of the next verse resurface. Takes the panic out of it. If the lines don't come back it makes it less obvious when you sing the last verse over again, or people think that's how it's meant to be. Dramatic effect.

A bit like the way you can get a single note improvisation instead of chords which is really a way of faking it while you work out what the hell key you are supposed to be playing in.