Adding an 's' at the end of he/she/it conjugates them to He's/she's/it's. This is for having "is" combined with the pronoun. Instead of "He is hungry," it would be "He's hungry."

Most Indo-European languages (I don't know about Hindi) have complex systems of conjugation. Most of this conjugation has been lost in English, and the addition of "s" in the third person singular is one of the only remnants of this system (the other remnant is the extremely irregular conjugation of "to be"). Compare this to some other Indo-European languages: English: German: Italian: Croatian: to go kommen andare ići I go Ich komme Io vado Ja idem You go Du kommst Tu vai Ti idi He/She/It goes Er/Sie/Es geht Lei/Lui va On/Ona/Ono ide They go Sie gehen Loro vanno Oni/One idu We go Wir gehen Noi andiamo Mi idemo Edit: I spent all that time trying to make this table look nice, and Italki got rid of the spaces when I posted! In some languages, this conjugation allows you to omit the subject while still making it clear who's doing the action. Note that these language also have different conjugations in different tenses, whereas all English verbs (except "to be") have exactly one form in other tenses. A similar conjugation system existed in Old English, but it's mostly disappeared. "To be" can still become "am/are/is/were/was" depending on the person and the tense, but otherwise the addition of the "s" in the third person singular is the only remnant of this old conjugation system.

It's the history. As a learner, you just need to accept it. If you want to look at the history, then you will be very happy to know that's all we have in modern English.

I'm not sure there's a reason... and if there is one, it might come from Old English or some Germanic root. In the simple present tense, adding -s / -es to regular verbs is the rule to conjugate the verb in third-person singular (he, she, it).

Hi Hunter, there is no reason it's just the way the language is built. I could just as well ask why is the past of do did? It just is, and that is what you have to learn.