Could anyone tell me what the practical use of some collections is? For example the keys from the Alcatraz Fortress? I have completed some collections various times and it seems I'm just storing useless items. Do you just complete those collections once and forget about them or is it useful to keep flying to, let's say, San Francisco and keep gaining more and more keys? I can see that some collections give you buildings, others give you bonuses. But some of them seem to be totally useless. Am I right?