from my understanding, keeping the answer to a few short sentences, this sort of behavior has to do with some evolutionary traits that should be deemed beneficial. one such trait is the fact that the open flower is used to attract some insects, namely bees, for the process of pollination. and most of these insects "perform their duties" strictly during the day time. also, many flowers send scents attracting these insects (along with their colors and the movement of the flower due to a breeze, as other means of attracting the attention of the insects). hence, the flower would benefit from closing at night to consume the limited resource of a scent when that resource cannot be used to its advantage.

hope that helps a little bit