The head of the beer, along with it’s traits, will be determined by the malt used in brewing, as well as any additives such as honey or fruit flavorings. The beers that are going to have the largest, longest lasting heads use wheat malts, so you’ll find that pilsners (such as this week’s Old School), Hefeweizens, and their relatives will be more known for their head than, say, a good porter.
Looking around I came up with a couple answers. First off is an obvious one: Appearance. Frankly the head of a beer adds to its appearance. It adds to the aesthetic value, giving a nice contrasting color and texture, plus it’s just nice to look at.
Secondly, and this is more important, is the smell. The head is formed by all the trapped gasses rising and releasing at the surface of the beer. Your taste buds are heavily influenced by what smell you’re taking in, so you could be missing out on half the experience. You should always smell the beer to get everything out of it.