When your website is listed at the top of the results for a search, that keyword acts as a free source of website traffic for you. 

If you have an advertising budget, you can place pay-per-click (PPC) ads on specific keywords. That is how Google Ads operates; advertisers bid for space at the top of a results page for specific keywords. 

Generally speaking, there will be more advertisers focused on a keyword that implies some sort of monetary action that will be worth paying for the price of the ad.

For example, the keyword “loans” has four advertisements at the top of the results, while the example above “what plants grow in the desert” had no ads.