Answer:
CSS separates the content that is in the HTML from the presentation aspects of the page(s) that is in the CSS.
Advantages are that it is possible to reformat multiple pages by changing one external stylesheeet. I often use three, one basic one that contains rudimentary formats for elements, a specialized one (when needed) that may be specific to only that page or a few pages (both with media="screen" defined in the link) and a print stylesheet (media="print") in the link. The print version removes the unneeded graphical elements.
Disadvantages are the ability to overdo the CSS. Very large ones or multiples can cause slow rendering as the browser must parse all the information. Badly built ones can cuases problems with the browser as it tries to figure out what is meant. As a rule I will not use the @import value or the universal selector (*) at any time. These seem to cause problems in lots of cases.
Since rules of specificity call for the last stylesheet to have precedence, repetious use of ID's and Classes's can slow things down.