Unlike grammar checker on Microsoft Word, Grammarly appears more advanced with not just spelling and grammar checker but also suggestions keeping in perspective acceptable English usage. There is also plagiarism checker that helps to spot duplicate content.
Grammarly is handy for both with English as second language and experts. For those with English as second language, it helps come with a copy that is accurate in terms of spelling while correcting so many grammar mistakes. While Grammarly might not be a replacement to gain an understanding of correct English grammar usage, still in a globalized economy, one cannot expect everyone to have proficiency in English.
For experts, it is not uncommon to have a few grammatical errors here and there because of busy schedule. Grammarly suggests where the software finds something erroneous or possibility of refinement. Being an expert, you take a second look if indeed there is an error or chance of a better copy or your initial copy suits the context.