Why is there a last call?

Ok, so I'm not sure how it works outside the US, or indeed in other US states, but here in Wisconsin, bars have to close at 2 am, 2:30 am on weekends, except for New Year's. I'm not sure the exact reason this law exists. Drunk Drivers? Noise disruption? Religious doubletalk? Antiquated carryover from prohibition? Thanks in advance.