These two pseudo classes are only working with links (<a href="...">), and with them one can change the look of a link on a page. All the styles can applied here, we can change text-color, text-style (underline...), borders, margins, and many other things.

In most browsers a link would look like this: I am a link. That wouldn't look good on pages that work a lot with design, say all text is blue anyway, it wouldn't stick out, or the background is blueish, or everything from text to images is in a reddish tone. Or just the personal preference of the site owner. Maybe he finds underlined links so boring, or he wants the color of the link to be the same as the rest of the text, whatever. There are plenty of reasons why one would want to change the looks of links.