Everyone above has given you the technical reason for the two mods, but I'll give you the practical reasons:
(Also: If I had it to do all over again, I'd have kept my Hypertech Programmer for WOT removal and tuning needs and had the dealer perform the 2 degree timing advance. I got better gas mileage with the Hypertech tune, but LOVE the 2 degree advance. Hypertech will not give us the 2 degree advance, so I sold it and bought a BDGT.)
WOT is what happens when you stomp on the gas while sitting at a light or stop sign. If you have a 2 wheel drive, you'll notice that it is sluggish off the line until you hit a certain speed (40MPH?), then whoosh, off you go. If you have a 4 wheel drive, the whoosh comes earlier (14MPH?), but it is still a bit sluggish off the line. WOT removal allows you to get the whoosh as soon as your tires hook up. Or if you're in 4 wheel high mode, off you go from the standing start.
The 2 degree advance is more for your running start. If you're cruising along at 60 and get pasted by a Chevy, you push the peddle to the floor and your T comes to life and over takes the Chevy. That happens sooner if you have the 2 degree advance. (Either way, you're still taking the Chevy.) The 2 degree advance gives the T a little pep while running along. Some say that it hurts MPGs, but my guess is that's probably because they keep punching it to feel the rush. After installing my Bully Dog GT and installing the tune as well as advancing the timing 2 degrees, I ran like that for a month. After about a month, I took the 2 degree advance off for one day. I quickly put it back in place the next day. If you're a person that loves to pass people on the freeway, you'll want the 2 degree advance.
So, there are plenty of options to get these done. Like I said above, I would like to go back to the Hypertech tune with the dealer 2 degree advance. The only reason I didn't have the dealer put the 2 degree advance on is because I wasn't sure if I wanted it at first. I had to try it out.