why we add s,es with third person singular
Middle English -(e)s (third person singular ending of verbs) from Northern Old English (Northumbian dialect) -es, -as (third person singular ending). Replaced historical Old English third person singular ending -(e)þ, -aþ (-eth). Possibly due to Scandinavian influence or related to -es, -as, second person singular ending of verbs.
inflection n : the change of form that words undergo to mark such distinctions as those of case, gender, number, tense, person, mood, or voice; a form, suffix, or element involved in such variation.