The X-wing fighters can change their wing configuration, from the X shape during battle to flat wings (the wings on the same side fold together) during FTL travel.
I'm guessing that the real world explanation is something like, "it looks cool". But what is the in-universe explanation?
There is no (little) resistance in space, so wing configuration wouldn't make that much difference. I can't see it making much change to performance, e.g. in angular momentum, either.