Okay, well this is actually super cool, thanks for driving my attention there. It seems that the word for this particular behaviour is "Nyctinasty
I think the short answer to your question appears to be “we don’t know”, because nyctinasty has been observed for millennia but there isn’t a clear, conclusive answer. In some plants that are open at night, it may be that they are pollinated by bats or nocturnal insects, but one of the open questions here is “does nocturnal nyctinasty happen via the same mechanism as diurnal nyctinasty?”. By that, I mean “does a nocturnal plant (like moonflower) use the same ‘muscles’ as a plant that’s open during day, like water lily?”.
I don’t recall it mentioning water lilies, but a book that you’d probably enjoy if you’re curious about this stuff is What a Plant Knows by Daniel Chamovitz. I remember there is some really interesting stuff in there about plant circadian rhythms, and the information is presented in an accessible and entertaining manner