In America's Test Kitchen/Cook's Illustrated they have a recipe for an almost no knead bread that has beer as one of it's ingredients. What does the beer add? And can it be replaced by something else (that's not alcoholic)?
The purpose of the beer (and the vinegar) in this case is to add some of the malty, fermenty flavors typical of longer-fermented or sourdough breads. You can either leave it out and replace it with an equal quantity of water or use a non-alcoholic beer
The carbonation from the beer might add a little extra lift at the start to establish some air cells and work the gluten a bit, but with an 18 hour room-temperature bulk rise, that benefit would be negligible. In most beers (especially large US commercial bottlers) there isn't enough active yeast left in the bottle by the time it is drunk to do anything either.
The author of the original article (Cook's Illustrated #90, Jan 2008) added the beer and vinegar in order to add flavor to a bread recipe that already produced decent bread.
I've found that beer in breads (and crusts, to be more specific) gives it a very light, airy quality. I don't know enough to speak authoritatively, but I'd guess the yeast (or whatever) in it helps its levity - which makes it so you don't have to knead it as much.
So I don't know for sure - but I'd guess that it has a functional purpose other than just flavor.