Those aren't "random" search terms. They tell quite a bit about the types of products you would be interested in, in this case hard drives and computers. Plus,

Those aren't "random" search terms. They tell quite a bit about the types of products you would be interested in, in this case hard drives and computers. Plus, data in aggregate is far more useful than data in isolation. Google knows what ads people with a similar search history have actually clicked on, and they can use that information to target ads at you. Google can rebuild most of the useful data from your Facebook profile (what kinds of books, movies, activities, etc. that you're interested in), and they also know what you're interested in right this moment. That is very valuable. I didn't know that they don't actually use historical search data, but they have the data and it can be used in the future. I don't see how using past searches to target ads is so different from using current searches.