Keywords are important in all aspects of online advertising. These are the words that customers search for, that you bid on, that trigger the display of your ads, and that become the focus of your ad copy.
To that end, you need to include your primary keywords in both your ad's title and body copy. That's because people look for the keywords they've queried when they're viewing search results. If those keywords aren't there, they figure the ad is irrelevant and ignorant. On the other hand, if you prominently feature these keywords in your ad copy, people will think that your ad is relevant to their search, and click for more information.
For example, if someone searches for "sunblock," she's going to scan the search results page for the word "sunblock." She's more likely to click an ad that contains that word than one that doesn't. The implication is that an ad that contains "sunblock" in its title or copy is relevant to her search and worth exploring further. An ad that doesn't mention "sunblock" isn't of interest.
For this reason, you need to write your ads around the keywords you purchase — the keywords that users search for. Include the same keyword you purchase in your ad's title, and maybe even repeat it in the lines of descriptive copy. Certainly include other related keywords in the descriptive copy. Your ad has to reflect whatever it is that the user is searching for.
Bottom line, ads that contain the same keywords that trigger the ad display tend to perform better than ones that don't. If you purchase a keyword, use it in your ad.