Translation:Hang the schedule back on the wardrobe where it was.
It adds emphasis. In English it'd be to the effect of "Hang the schedule...where it WAS." Russian has much freer syntax than English, so English tends to stress things with intonation. Russian uses intonation as well, but they also use particles as a form of emphasis, whereas in English, I can't think of any examples of this phenomenon. Someone correct me, please. I'm sure we do it.
The "и" here is not a conjunction "and", it's a particle; in this case, the presence of "и" implies that the listener might know where the schedule was. Without an И before the verb "было", the sentence has almost the same meaning, only that the listener might want to find out where the schedule used to hang, as it is no longer so explicitly shown that "on the wardrobe" is known information.
I believe perfective are used for actions that you're not thinking of as processes; imperfectives then are for processes, states, etc.
So if I "was eating an apple" or "ate part of an apple", there was a process of apple-eating I engaged in, perhaps with beginning and end points, or maybe it continues; Russian would use the imperfective verb for this. Meanwhile, if I "ate (up) an apple", the verb isn't concerned with the process: the natural extent of apple-eating was completed; this is where Russian uses perfectives.
Think of verbs like "begin", "end", "find out", "hang"*: there's usually a singular point in time where the action happens: one moment the action is in the future, and the next instant it's in the past.
*in the transitive sense "The man hung the picture;" not the intransitive "The picture hung on the wall."