Because things will go wrong, and when they do, you're going to want helpful caterers. Let me illustrate:

Picture a bride (me) standing in the "ready room" getting ready to make my grand entrance. I was nervous, my parents had walked down the aisle ahead of me, and I was waiting for my queue. I was supposed to be escorted, fluffed, primed and prepared by the staff at our venue, however, said staff was nowhere to be seen. It was just me. I was due to make my entrance via freight elevator, (the huge manually operated kind with the big metal doors and pulleys and stuff). So I had to wait. Still no staff, no fluffing. I stood there with my nerves building, thinking about all my friends with their necks craned, waiting for me to come down the aisle. Finally, in a moment of panic, I asked a server working for the caterer what to do. In the midst of clearing up wine glasses, she looked around, there was no one. Dropping what she was doing, she told me not to worry. She straightened my train. She took a loose piece of my hair, and gingerly pinned it back into place. She told me I looked beautiful. And lastly, she manned the elevator. I don't think she really knew what she was doing, and I have no clue how she figured it out, but she did it.
So here's to the lady in the black pants and the white shirt! If I could be there to fluff her skirt and man her elevator, I would.