beer can chicken- What purpose does the beer serve (other than you get to drink some before grilling?

You are correct. The beer can was traditionally used because it allowed the chicken to stand vertically and the liquid in the can would boil and infuse the cavity of the chicken. I have a "beer can chicken pan" that has a place for putting herbs and liquids and for the chicken to mount over the liquid container

The liquid in the beer marinates the chicken as it cooks and keeps it moist. You don't have to use beer. Pick a liquid that you like. Try a Sprite or something like that. Lemon/Lime is good on chicken.

Some people just prefer the taste. Kind of like basting meat in wine, or drinking wine with a meal. White wine gives fish and chicken a more rich, robust flavor while red wine gives red meats a rich and robust flavor.