CSS - explain

Ok, I know what they mean, and what they do - for the most part. I have done my homework as far as how to use them and such and even worked with them in Dreamweaver a bit.

My question is - what do you use them for? I mean are these style sheets really only for huge websites where there are pages and pages of info that you need formated quickly or do you think it’s worth it to use them for small sites, like 5- 10 pages worth.

The reason I ask is, I havent really ever used them before and would like to start. I do all my work the old fashioned hard way and I just wondered if this will be that much faster for smaller scale sites as well as large ones. What do you think? What do you use them for? Are they a Neccessity in web design?