Same reason we have money, instead of a barter economy: it makes for convenient shorthand.
Labels work fine for a lot of things ... so long as we don't hold on to them too tightly.
It can go too far, of course ... to the point where the labels supplant the ideas themselves. Just check into one of those Calvinist threads on premillenial dispensational whatsis-ism and you'll see the point.
I understand the money part, but I think it leaves too much chance for stereotyping people and generalizing their beliefs. I agree that they can go too far.