Because da is the conjugation of dar for el, ella & usted. In English virtually every verb ends in an 's' when conjugated in the third person he/she/it form. (I give, they give, we give, but he/she/it giveS).

Da is the conjugation for 3rd person singular (él, ella, and usted) and also the singular imperative, or command form. Either of those translations should work in those cases, but the imperative will usually be formed as 'dame,' a combination of the imperative/command and the indirect object pronoun, and will also rarely have a subject in the sentence because the person you are commanding is implied.