You are not guaranteed that you can apply a code to an existing reservation. For example, if you have a courtyard view room at WL booked, room only, no discount, and an AP code for your travel dates comes up, they may have availability at WL and they may not. They may only have availability for woods view rooms, but not courtyard view. So you might have to change room types or resorts to get a discount. I paid rack rate last December because I had booked a specific room type as a treat to me, and when the AP and resident discounts came out, there wasn't any availability for my resort room type. However, in February, I was able to have a code applied to a previous reservation, because there was availability at the resort and for the room type I had booked.
Some people are all about the discount. They aren't so picky about the resort or view if the price is right. But sometimes you really want that water, savannah, or courtyard view, and are willing to pay more to have it. Most of the time, I fall into the latter category, but not always.
It is easier to modify a reservation made by phone than one booked online. I only book online for my last minute trips that I know I won't change.