The reasons are mostly for conservation of energy and to have fun. I like changing a do si do to a swing or something else because I find that call awkward in its original definition and not pretty in its execution. I enjoy doing it. You make the statement that doing your own thing is alright as long as you end up in the right spot. In my view, how I and the rest of the set get there is as important as getting there.
Square dancing is a team sport.
It's success depends on eight people cooperating together to dance a pattern. More over, this same pattern and cooperation is being duplicated by perhaps dozens of squares on the same dance floor. On the occasions when we all succeed together, you realize that you are part of something bigger than yourself and most people who have experienced it get a great deal of emotional satisfaction from the experience. The first time you short cut a dosado or refuse to promenade, you have just screwed up the team and my enjoyment of the team effort. It's like someone deliberately playing a sour note at a band concert.