Ok First off, assault weapons is a term created by the media, "assault weapons" mean they can be fired in full auto.
Now, with semi auto rifles, there is really just difference in the "extras". The pics of my guns, the wooden rifle on the left fires the same round, and just as quickly as the rifle in the middle. They are both semi automatic rifles with detachable mags. All the other stuff is just little extras that some people want: the ability to put scopes/sights, lights, etc on a gun is something some people want. The "scary" looking stuff like adjustable stocks and pistol grips, just make guns more ergonomic and comfortable to shoot.
A lot of people hunt with what are popularly referred to as "assault rifles". Some people just like those types of guns.
The question of why someone owns guns is one that a lot of gun owners might be hard pressed to answer. Some use them for hunting, but for a lot, its just a combination of they're fun/i can/i hunt/protection from criminals/possible protection from tyranny someday/i just feel better owning one
Ammo is pricey, but it can be combated, some people reload their own rounds, and buying in bulk instead of at walmart helps a lot. I last paid about $.17 for 9mm, and about $.23/round for .223. You can get about 500 rounds of .22 for about $15.
As far as firing them, its not really about the rush, its more about focus, just like skiing. Thats another thing that keeps ammo costs down, is you're not just blasting away, you're aiming at stuff and trying to improve.
And about the model names, how do you remember all the different skis out there? It is an expensive hobby, but so is skiing. So are cigarettes.