1 - It is the "right" way to do it. HTML is for structure, CSS is for presentation. If you're quality-conscious, it matters. Sure, I can open the door by kicking it, but isn't the doorknob there for a reason?
2 - Using CSS, a site can be redesigned very quickly. For a personal site, it means less of your precious time taken in converting all of your pages to the new design. For a company, it means less money spent converting to the new design. Redesigns are inevitable. Why not save yourself some pain in the future?
3 - Alternate presentation. Instead of designing a "text-only" version of a site for accessibility, and a WAP version for people on phones and PDAs, you can create alternate style sheets. Same exact content, different presentations. Check out the style switcher on my blog for an example of alternate presentation -
http://blog.grayscalecms.com/
4 - Lighter pages mean increased usability. I once worked on one of the highest-traffic websites in the US Government. The original design was a tangled mess of tables and spacer images amounting to 173kb JUST for the template. I converted the site to CSS, retaining the visual appearance completely, and took document weight down to 56kb.
The argument that browser inconsistencies create difficulties with CSS presentation is bulls**t. Take a trip to
http://csszengarden.com/ to see what I mean.