Why do we have an onLoad event and no onUnload?

Hi all,
sorry to ask something that you may have discussed already some time ago 
before I joined, but I don't quite understand what the use case for the 
onLoad event is.

If it is to initialize resources on behalf of the user why is there no 
onUnload event to release these resources?

Are these events useful at all given that the delivery is not guaranteed?

Stefan