Can/could

You would never respond by saying "I can eat" which has no meaning beyond the literal.

"I could eat" is an idiom meaning roughly "While I'm not hungry enough to actively search out food, I am hungry enough that if you [the questioner] want to get something to eat, I am willing to go eat with you."

I can eat just says "I have the physical ability to lift food to my mouth, chew it, and swallow it". It is therefore inappropriate as a response to Are you hungry?

I could eat, as a response to Are you hungry? says "I am hungry enough to eat if you're thinking of offering me something to eat, but not extremely hungry, so I won't starve if you don't offer me an