Got my first flight from LAS-LAX tonight with Southwest (BS A2 apparently)and have been reading through a few threads regarding boarding, seats etc and have noticed quite a few people referring to WN. Why?

Also, when I booked the flight, it gave me a choice of seats and I chose some random seat near the front. Can't seem to find any reference to this in my booking and it seems that it's just "first come first served" so why did I get to choose a seat in the first place?