Here’s what consumers have asked with answers from Grammarly staff and previous consumers.
One of their people could answer, but in short, accepting all is mostly a bad idea. Writing is contextual and Grammarly is more literal. It will flag many things that are not errors. The online report is very interactive. You can click down through each error and quickly accept Grammarly's suggestion or elect to ignore it. It is fast, and by seeing our own errors we learn to improve our own writing. A program that accepts all revisions is a program that thinks for us--never a good idea.
No almost thing is free in the public services, especially when it pays some millions in advertising!