Jeez this has been discussed over and over again for thousand of times already. Here you go - it's a bit general but it's a good article: http://bigsquaredot.com/blog/2007/03/20/explaining-css-to-your-clients-with-five-simple-answers/
Plus, it helps with SEO-friendly indexing of your web site. And, as mentioned in the link from the previous reply, it will help make your web site semantically correct -- which means that screen readers and other devices will have an easier time interpreting your web site's content. However, if none of that convinces you, and if you really don't care about any of it, stick with tables. (Not really, but you get my drift, right?)