It makes it more expensive to buy alcohol to drink at home so people say the heck with it, if it's not saving me much money to drink at home I'll drink in the pub.
Think of it like food. If you can cook and eat at home for $3 and the local restaurant charges $15 then you'll likely cook for yourself. If the government puts a minimum price on groceries where the ingredients now cost you $10 then you'll likely say the heck with, I'll eat at the restaurant where I don't have to cook and clean, and I can flirt with the waitress.
No. It just creates a black market and actually destroys profits for the big companies. Do not presume that the large legitimate brewers and distillers are the sole suppliers.