You can make money from them. I believe the visitor has to click twice, once to get to google then once on the ads. However, if you click through to google, you might click on many different ads.
It can be beneficial... if someone doesn't find the information they need on your website they'll use your search rather than backing out to search engine.
To take this one step further: Keep track of what they search on and it will provide you with many ideas for additional related content that you can add to your site. You will know what REAL visitors are looking for and you can tailor your pages to it, thus getting very targeted traffic.