Most search engines use search term frequency as a primary way of determining whether a document is relevant. If you're researching explosion risk area and the words appears multiple times in a Web document, it's reasonable to assume that the document will contain useful information. Therefore, a document that repeats your words over and over is likely to turn up near the top of your list.
If your keyword explosion risk area is a common one, or if it has multiple other meanings, you could end up with a lot of irrelevant hits. And if your keyword is a subject about which you desire information, you don't need to see it repeated over and over. It is the information about that word that you're interested in, not the word itself.
