MS Word and other text redactors may highlight certain spelling mistakes, but they do not check your text comprehensively. However, spelling is not everything. Professors or potential employers may be dissatisfied if you misuse words. Grammar checker online has a lot more to offer. It goes beyond spelling and checks whether the actual words you use match the context.
An essay grammar checker is a tool designed to check your text in its entirety. It analyzes words and sentences based on the general context. You'll get a better analysis of each writing piece. Such software is being used even by native speakers as we all can make stylistic and word choice mistakes when writing. Free checkers are a great solution for all authors.