It's a relic of a bygone era. The theory is to make it more inconvenient to by alcohol so people will buy less of it. And then, only highly trained and responsible people can sell it. Anything over 5% has to be sold be the Commonwealth.
Beer (with a few exceptions) sold by the case, with the theory that having to buy more and spend more, will discourage people from causally buying it. I think it just encourages people to drink more.
We have awful alcohol taxes, not as bad as that though. They're called the Johnstown flood tax. It was to pay for the Johnstown dam breaking (twice) and destroying the town (twice). Nevermind that that was like a 100 years ago. Most people on the eastern side go to Jersey or (even better) Delaware to buy alcohol.