If I may, I'm going to cite the British Council (and this specific page).
Present continuous is used:
- To talk about the present in the following cases:
→ for something that is happening at the moment of speaking
→for something which is happening before and after a given time (At eight o’clock we are usually having breakfast)
→ for something which we think is temporary (I’m working in London for the next two weeks.)
→ for something which is new and contrasts with a previous state (These days most people are using email instead of writing letters.)
→ to show that something is changing, growing or developing (The children are growing quickly.)
→ for something which happens again and again: (He’s always laughing.)
- To talk about the future in the following case
→ for something which has been arranged or planned (I am going to the city this afternoon)
Using "I eat" does not clearly carry a meaning of "I am doing the action of eating".