What's the deal with foam?

It depends on the style. Some styles a long lasting creamy head is desirable while others little to no head is desirable. For example a Pilsener is supposed to have a nice creamy head that sticks to the glass while a barleywine has little to no head.

Basically it's appearance

 

I'd prefer the space in the glass that the foam is taking up to be replaced by more beer.

 

listen to the BeerSmith pod cast Head Retention with The Pope of Foam.

http://beersmith.com/blog/2011/09/28/head-retention-with-the-pope-of-foam-beersmith-podcast-23/

this answers your question in way too much detail but you'll never look at foam the same way again.

 

Lacing looks badass, especially when it consists of an abundance of closely spaced parallel bubble lines encasing the entire interior of a glass.

 

Also, lacing on a glass is an indication that the glass is clean. The lacing sticks to the parts of the glass that do not have dust, dirt or oil on it.