The GPS in my car is names GlaDOS, after an evil computer in the games Portal and Portal 2. GlaDOS uses the promise of a reward, cake, to lure her victim, you, into one dangerous situation after another. Just like GlaDOS, my GPS appears to be helpful, but will kill you if she gets the chance. She often routes me right past intersections with lights and turn arrows, and then demands that I make a left turn at a dangerous intersection with no traffic lights and a stream of oncoming traffic.
In truth, however, my GPS is not evil--she's just programmed that way. She calculates her route based on the shortest possible distance, and simply doesn't have the computing power to weigh the safety of one intersection against another. All roads are the same to her, so she is just as likely to route me down a narrow, two-lane, shoulder-less road as down a four-lane highway. I can't put my brain on hold while using the GPS. She's a tool, but I'm the one who thinks.
Spell-checkers and grammar checkers are the same way. They are programs, but they are tools, and you can't put your brain on hold while using them. Often, a spell-checker will not distinguish between a word and its synonym. I once read a story where the characters were horrified to come across a scull. I tried to point out to the young author that she had misspelled the word, and she assured me that the spellchecker had said it was right. Well, scull is spelled properly--if you are talking about a small boat.
The spell checker might help you find misspelled words, especially if you are slightly dyslexic, like me, but it can't replace your brain.
Grammar checkers are worse, I've found. Grammar checkers don't like stylistic writing. An author may use sentence fragments for emphasis, and when writing dialog, both sentence fragments and bad grammar can be deliberate. When the word processing program calls your attention to everything it considers an error, it becomes easy to turn a blind eye to it. The Little Grammar Checker calls wolf once too often, and then you miss something you should have caught. Grammar checkers do not replace proofreading.
Another problem with grammar checkers is the ability to miss grammar problems. For example, in Word, the error in this sentence is caught: "Although the great Depression was the most severe, there has been others." A wavy blue line appears under has. But in this sentence, "Programs like this in the New Deal led by President Roosevelt helps the U.S. recover from the worst depression in its history in 1933," the error goes right past the spell-checker. Although the program can look for subject-verb agreement, or see that a helper verb is the wrong form, it can't see that a sentence should be past tense. Computers are tools, but you should never put your brain on hold.
The cake is a lie--never trust a computer to think for you.