Whats the main difference between monitor systems and PA systems?

I hear lots of talk about small monitors such as those made by krk and pioneer as well as the small 8 to 12 inch subs that you can couple them with. I was looking at a 10 in krk sub and saw that it cost around $400. My question is, for that kind of price, what is the advantage to these smaller monitors when you could get a full size PA system such as a behringer b215 and a b1500d subwoofer for a similar price

summary for those who hate to read:



VS



and why?