CSS does a lot of things. It gives you much more control over text and images. You can affect leading, margins and things of that natue that you have very little control over in standard HTML.
In terms of being a timesaver, you can set up various styles for different tags. You can make specific styles for Block Quotes, Body Text, Captions etc. And you only have to set the font/size/color/margins etc. once. You can also save these styles in an external file that all of the pages on your site can access.
It does a lot more than this, but that's the gist of it.
If you use Dreamweaver there's a style pallette. Play around with that. I don't know any good websites for it, but I will say that it's very simple to learn, nearly as simple as basic HTML.