Among the many odd things about standard varieties of English is the “s” at the end of “knocks” as in “She knocks on the door.”
If you were to change “she” to “I,” “you,” “we,” or “they,” the “s” would go away, and “knocks” would become “knock.” Why does third person singular tense get an "s" tacked on the end?
Frankly, the “s” at the end of “she knocks” or “he sings” or “it talks” isn’t doing any work. It may be considered grammatically correct, but it’s also redundant and unnecessary.
English Professor Anne Curzan often describes the history of English as "the history of endings falling off." This is clear in the history of verbs.
In Old English, you'll find that in all persons, both singular and plural, there were different endings for the verbs. If you've read Renaissance literature, you've likely seen some of the endings that were still hanging around: "she knoweth," "he hath," "thou sayest."
Those are remnants of the Old English endings of verbs falling off. Those endings are then replaced with "s” which never falls off in standard varieties of English. However, in many varieties of English, both in the United States and around the world, the "s" has fallen off, and that’s where we see things like “she knock,” "he sing,” and “it talk.”
Somehow, the "s" at the end of third person singular present tense verbs has managed to hold on in standard English, even though leaving the “s” off is a much more regular and logical system. Curzan says that’s why we should be careful when people try to argue that standard varieties of English are somehow more logical than other varieties.
"Standard varieties are standard because they're the varieties that got institutionalized in schools and by printing presses. It's not that they're more logical," says Curzan. “In fact, you can sometimes find much more logical constructions in non-standard varieties than in standard varieties."
Be sure to check out this article by linguist Peter Trudgill for more oddities of standard English.