Then, sometimes, there's a reason. This, that, or the other thing needs some build-up; if the characters don't get to know each other, say, we can't have a scene where they do not act as total strangers.
OTOH, sometimes I have put stuff down as in the indeterminate future when there's no good reason not to have it now. And the question about whether to put stuff here is "Why not?" not "Why?" Because you don't want to put off all the good stuff to later. You want to have good stuff every step of the way.