The answer to the first bullet. The roll of the CV or constant velocity joint is to enable movement with independent suspension. As the geometry changes (when you hit a bump), the wheel moves up. If the axle were solid, this would be unable to happen. The CV joint allows for the movement of the suspension. I guess you could think of it like a swivel socket.
As far as replacement of the joint, if you want to do it yourself Autozone or O'Reilly's (most autoparts houses) loan tools for free, including axle nuts. Obviously, you will need a jack and jack stands. Hopefully you have an impact gun, that would make the job that much quicker. Otherwise, I would plan an afternoon. With pneumatics I can do this job in 20 minutes. Also, I would replace the entire axle as it makes the job much simpler and straight forward. I'm not exactly sure what size sockets you will need, but you will need a breaker bar for the axle nut and a hammer to separate the ball joints. Things to look out for would be damaging the break line or ABS wire if equipped. That could happen if you just let the knuckle dangle after separating the ball joints, so you may want to make sure that doesn't happen. I personally use zip ties and tie the knuckle up to the upper control arm.
All in all it's a very simple job and good one to do as it will give you good experience with hand tools. Difficulty rating from 1 - 10, I would give it a 3.