Why do we add 's' to verbs in the third person singular (he/she/it)?

Why do we add 's' to verbs in the third person singular (he/she/it)? "I speak" vs. "She speaks" I know we do this to make it grammatically correct but what is the reason behind this rule? And if one redundant information is gained, why only do this in present tense? "He spoke" not "He spokes"? Redundant in the sense that subjects like (he/she/it ...) are never omitted when use with s-conjugated verbs anyway.

In French verbs conjugation is even more complicated but there is a reason. Ex. je parle (i speak), tu parles (you speak), il parle (he speaks), nouns parlons (we speak), ... Because in the past the Romans omitted subjects so extra information is added to verbs. But does this or any kind of similar reason apply to a language of a different family like English?