Because spell checkers only do 80% of the job.
If you, like me, speak English as a second language, you know that using correct spelling doesn't protect you from writing those awkward sounding lines.
Tell me, how many times did you come up with two ways to say the same thing, and couldn't decide which one was the best fit?
My solution was to google both expressions and check out the number of web results.
Low figures meant that very few people ever phrased the sentence that way, thus it was probably incorrect.
On the other hand, higher numbers indicated common use, and the 3 line preview in the results helped me figure out if I was using the right form.
This tool does just that, in a much quicker and convenient way.
Just please remember that this in NOT intended to be a SEO tool.
For any question, contact me!