Easy.  They improve your web site's ranking in Search Engines.

Most search engines include Meta Tags in their measurement of the relevancy of certain keywords as they relate to your web site. Some read Meta Tags and post them with your listing. Other engines review the tags along with the other textual content of your web site to measure overall relevancy. If you are planning to submit your site to any of the large search engines, your content is the most important "keywords" you have, but Meta Tag implementation is also essential! Start by making a list of 25 keyword phrases that best describe your business in the most detailed way possible. Think about what makes you unique - that is the key to success on the internet.

Choosing Your Description & Title

The description of your page is usually what the search engines will show the searcher. It basically should summarize your site, and include as many of your keywords as possible. Your title is basically the same thing, but a much shorter version. Be specific with your titles, they sometimes serve as keywords, depending on the search engine.

Common Words
Avoid words which have a lot to do with the Net in general. When a user searches for one of these words they get a huge number of results and yours is likely to be way down the list.

The purpose of keywords is to target words that will bring people to your site as opposed to other people's. Since nearly every site on the Net is to do with 'web' or 'Internet' or 'services', using these words to target your audience is pretty pointless. Most of the robot-driven engines actively ignore common words like this.

Also, avoid using general words like "Cars".  Be specific, "Used cars" is a much better choice. " Used Hondas in Los Angeles" is an even better choice.

Keyword Phrases
These days there are so many pages on the Web that users usually search for 'strings' of at least two or three related words at once, for example "california carpet fitters". Now, if you have all three words on your list separately, you'll be listed below somebody else's page if they've targeted all three words together, in a Keyword Phrase. In the above example you would treat "california carpet fitters" as one word which just happens to have space characters in it.

Search engine stuff is a big subject but, the key to good rankings besides submitting is having repetition of keywords in the content of the web site. This means the text, image names, titles of the pages, meta tags, and alt tags. They all must work together to create something the search engines decide is “relevant”.