Bank Deposit Cut-off Times

Why do banks have a deposit cut-off time for it being posted to your account that's like an hour before they close? I got up to the windows about 20 minutes after cut-off time and said "I'm to late for this to post today and that I needed to cover some charges". The teller said "just a minute, let me see what I can do". She proceeded to ask the other tellers "if they had gone next day yet?". From what I figured out, at 4:00 they change out there cash drawers and all the other transaction stuff and start with a new drawer for the last hour they're open. Is there a legitimate reason for this or is it a ploy to suck more money out of some of there customers in overdraft fees? I could go on and on about overdraft fees an how they post things to your account to maximize them, but I'll save that for another day. I want to hear your comments on this!