Two questions:

1. How are tableless designs achieved?
2. How would one benefit from these?

Tableless design asre acheived by using CSS to add style to your (X)HTML documents. There are a lot of reasons to use CSS layouts over tables (accessability, search engines prefer it, more control over your layout etc) Have a look at some of the previous posts here at WT. There's a great number of posts of people explaining the advantages (and rarely, the disadvantages) of CSS.