Why did we add a "S"to the end of verbs in the English language?

Dart is right as far as he goes. English verbs were once as specifically inflected as those of German or of most Romance languages--four to six forms in each tense, three singular and in some languages three plural. Old English (the English of 1000 or more years ago) had four, and the third person singular ended in -th--as it still does in the King James Bible, translated in the early 1600s. Some time over the next century or two that th changed to an s. I'm sorry that I can't say specifically when or why, but the Declaration of Independence (1776) begins, "When . . . it becomes necessary," not "becometh." Meanwhile, the endings of the first person singular and the first, second, and third persons plural simply dropped off. (The second person singular is simply no longer used in standard English except in prayers or other religious speech. It's "thou," and its verb usually ends in -st--except for "thou art.")

Because that's the third person singular inflection for a regular verb in English.

To show present-tense forms of a verb. All languages do it.

It was originally "P" but it was too hard for people to pronounce