A large audience for these types of tools is people for whome English is not their first language (this includes me, so please forgive any spelling/grammar mistakes you may find). This audience may be very eloquent and stylistic in their native language, but learning languages (especially when you have passed your teens) is hard, and sounding professional and sleek requires a lot more effort than you might think.
Here's where Grammarly comes in. It reformats your sentences to be more consistent with your projected attitude, removing a ton of work for some people.
I know plenty of people for whom the tool is really useful -- my foreign-born wife for example. But were she to limit her native expression with the same kind of "simplifying" that the tool offers, I am sure she would not be perceived as having any kind of personality at all.
For me, when I write -- and I like to write -- I choose to break convention often. The tool frustrates me and wastes my time. Sure there is an audience for this, but I don't want to sound like everybody else. That's MY attitude.
...and reading the comments does make it sound like a key-logger ... so no, I can't say I trust the extension.
And obviously the lettering is a lyrical joy.
10/10