Why do it anyway? It is to promote good standards for writing (x)html and using the web standards. Dustian Diaz has this on his site,
The idea behind this event is to promote Web Standards. Plain and simple. This includes proper use of (x)html, semantic markup, a good hierarchy structure, and of course, a good ‘ol play on words. It’s time to show off your <body>.
That sums it better than I can. For me, wordpress is taking care of it. I gladly support better markup and semantic correctness that we can try to achieve in the future. Not all structures have meaning, but at the very least, we’ll try.