For example if you're writing a time travel novel, and you want your 19th Century natives to sound different from you contemporary time travellers, one thing you can do is look up greetings like “Hello” and “Good morning” to see when those terms were most common. It's also handy for concepts that change over time; I tried searching on mental illness, madness, insanity, and dementia, and could instantly see which terms were popular and when. Ditto slang like okay, groovy, and douchebag. You can also check not-quite-correct usage like “alright” and see that that spelling was not seen before 1960.
In fact, the 1960s seem to have been a pivotal year for a lot of things. Try searching on some dirty words and you'll see what I mean. Go ahead and have some fun! It's free.
And don't say I never gave you anything. -)