I use Google Trends for extensive keyword research.
For example, if I'm doing SEO for a new online store, I'll take all my main keywords for each category and run them through Google Trends.
From there, depending on the period analyzed, I can figure out if the keyword is on a down spiral or if it has a full upwards trend and it will be relevant in the future.
Another cool thing Google Trends ca reveal is buying seasons, if you spread the graphic on a long enough period you will notice certain spikes at some point of the year. Those are really important to figure find and keep in mind. Because you can start preparing extensive marketing campaigns for those high-interest periods.