When well made, the foam should actually be a "microfoam" comprising a thin layer of tiny bubbles that actually have some coffee flavor and a hint of brown visible. The only pure white should be the little artistic zig-zaggy flourishes great baristas put on for visual appeal.

I read somewhere that drinks can be too big if you want a stable microfoam layer. That article recommended that to get a good cappuccino out of Starbucks, you should ask for the unlisted 8oz "short" rather than than the 12oz tall. The larger sizes are ridiculous and meant for people who like milk rather than coffee.

I think the microfoam layer serves two purposes.

First, a good microfoam layer actually tastes good. The texture adds some variety, by way of an opening act, to the base liquidy texture below, the main act. For a well-made drink, the transition from opening act to main act is smooth. You won't have the annoying effect of hot liquid spilling out into your mouth from under a too-big foam layer that you cannot get into your mouth at all, as you tilt the cup. I hate those messed-up drinks. In a good drink, you'll have a nice blend of foam and liquid coming into your mouth in the first several sips, at which point the foam will have dissipated.

Second (and this is a theory), I think it may help keep the drink warm longer, by creating a trapped layer of air that blocks both conductive and convective heat transfer to some extent.

There is also a different kind of foam, a "pure" coffee foam, called crema, that naturally comes out with espresso. That's different - in that case, the crema is merely a sign that the espresso shot is a good one.