It reveals your reading level. Writing towards the 4th-6th grade level makes reading what you write easier on your readers (see below).
It reveals the reading time of your post. I personally strive to write in snippets that can be read in less than 3 minutes.
It reveals keyword density. If you want to be found in Google, keywords are important. It's also helpful to avoid being to repetitive.
It will read your writing back to you. When you hear what you wrote read back to you, you'll usually find ways to improve it.
It works with Grammarly. Google Docs doesn't, which is why I prefer to write in WordCounter.
When you are done writing, you can save it to Google Drive (this feature however seems a bit buggy, so proceed with caution).
Here's the important thing about reading grade levels: It's not about education. It's about ease of comprehension. Low grade level copy is not "talking down" to educated readers or treating them like children. Think of it instead as a form of courtesy, like enunciating clearly when you talk. The most super-educated PhD. will appreciate and respond to copy that's easy to read. Copy that's 4th to 6th grade level is barrier-free and can be read quickly and comprehended well by all adult readers. Go much above that and it takes extra concentration. A reader who's in a hurry will have trouble comprehending, and will be more likely to skim or to stop reading. And it just gets worse as the grade level climbs. Source