Online grammar and proofreader has limitations in terms of number of words. So when the grammar and proofreader is plugged in on the microsoft word it simplifies the process while making grammar check. Is matter of fixing the text of the paper some mistakes before send to the reviewer.
Grammarly is a good free option. If you're looking for something more then I would recommend proofstep. I've been using them to proofread my work for a while now. www.proofstep.co.uk