You want to know how many people are searching for topics and products related to yours, not only to size up the potential market but also to see if the terms are trending upwards or downwards. It can also be helpful to compare and contrast competitors, complimentary search terms, or other wording that may be of interest.

Google Trends lets you see trends for specific search terms filtered by location, time period, category, and web/image/news/shopping/YouTube search. It lets you compare interest over time of multiple terms against each other on a relative scale, regional interest, and related searches.