Paste up to 500 words and the checker returns suggested corrections, each highlighted in place. You accept or ignore them one at a time rather than having the text rewritten around you, which matters if the point is to learn the pattern rather than to hand in clean prose.
It checks grammar, spelling and punctuation. It does not score your writing. TOEFL and IELTS graders reward things a grammar checker cannot see — whether you addressed every part of the prompt, whether your reasons are specific, whether your register suits the reader. A response can be error-free and still sit mid-band for those reasons.
Write your response under time first, then check it. Checking as you write trains a habit you cannot use on test day, where nothing flags your errors.
Read each suggestion before accepting it, and note which ones repeat. Most test-takers have three or four recurring errors rather than thirty different ones, and the repeating ones are what a study session should target.
For what the graders actually reward, see the TOEFL Writing rubric guide or the IELTS Task 2 guidance.