Humans are very "image" based, and text can very descriptive - but generally not very "memorable" as a symbol, and sometimes doesn't supply enough chance to fully explain what a certain function does. You want to grab their attention as quickly as possible, and make it easy for them to locate and remember an item when visiting the site again.

For example, RSS feeds have a little orange icon.   It's recognised very easily these days. If you were to just put "RSS Feed" on a page, people would easily skip past it and not realise. Putting a small icon next to it tells the user immediately that it's an RSS feed (as long as they know of the symbol/icon).

Some icons, for example - a small "house/home" symbol generally means "Go to the homepage" in most peoples languages and also overcomes any potential language barriers. If someones eyes are flicking around a web page, they'll be drawn to "images" first and if that image can portray an actual meaning or function to the person without needing to read the text, then you've made things a lot easier.

A lot of download buttons now are very large and with green arrows pointing "down" to re-enforce the meaning - it's a growing trend and helps many users immensely in quickly locating what they are after.