Actually the OP specifically said “grammar” checker, which since they can write is likely an intentional distinction. Alternate phrasings reveal the absurd elitism of the statement.

“I don’t need a spellchecker because I can spell.”

“I don’t need a calculator because I can do math.”

As a longtime copy editor and proofreader, I would have to agree that spellcheckers and grammar checkers are not worth shit.