The general idea behind cascading stylesheets is to save time when changes need to be made. If you use inline styling in your product descriptions, for example, if you want to change the font size, you'll need to go to each product setup page and make the change. If you've used a <div> with the same id or class name for all descriptions, it's a ten-second job to change them all in the stylesheet.
I think it's also a good habit to get into, since you learn a lot about CSS by using the Zencart stylesheets and it kind of weans you away from the flat HTML page mindset by thinking about the site as a whole.
I do use inline tags when I want to make one or two words bold, for example.