Weather can reduce flow rates because you can’t see at 150 mph to take off or land but some severe storms en route can require surprisingly long detours as well.
And then there are the knock on effects.
A delay early in the day can mean that a crew can run out of hours. A 3 hour delay for mechanical reasons on a Gatwick/Dallas flight once led to me experiencing a push back followed by an immediate return to gate because the crew realised they would be out of hours by the time we got to Dallas so we took another 3 hour delay while they rounded up a new crew.
One day I will walk up to the gate and tell them that his is no reason, but only a symptom of another reason which they don’t bother to tell their passengers.