Yo me comí

Yo me comí
 

I was doing some practice, and I came across the translation of "I ate the rest of the cheese" to "Yo me comí el resto del queso".

Why does "me" have to be there? I understand when me is the object of the sentence, for example, "He helped me" = "Él me ayudó".

But I just can't figure out what rule requires "me" in the other sentence.

The verb comer is pronominal in this sentence. That explains where the 'extra' pronoun comes from.
comerse = gobble up