When you want to say who does what.
Good news! The present tense in English is easy! ☺
There are only two choices when you want to say 'who does what':
1. the base (simple) form of a verb, 2. the base form of a verb + s (use the second choice with he and she but use the first choice with everyone else.
Base Verbs: eat, drink, walk, read, come, drive, say etc