The present perfect refers to an act that took place in the past, from the perspective of the present. "I have eaten" means that at some point in the past, eating occurred. Now (at the present), it is over.
The simple present tells you about what's going on currently. "I eat" means that the act of eating is ongoing. (Though it can also refer to a habitual act; I may not be eating right at this instant, but it's the sort of thing that I could be doing right now, because the eating isn't complete.)
