Help please, I am finding this section a little difficult to grasp.
. . . why comer. This is about 'we' = vamos, so why didn't that carry to 'eat' with = comemos.
Think about it this way: In English, the Future Phrasal for "What are we going to eat?", has the infinitive, "to eat" with the "going", as in "going to eat". So perhaps it is not too surprising when you think about it, that in the Spanish Future Phrasal, we have vamos (going) + the Spanish infinitive "a comer". So, we have "vamos a comer", just like we have in English, "going to eat". :-) So the Future Phrasal, both in English and in Spanish, follow the same formula: "Going + infinitive of the verb" to mean "Going to do something in the future". That is why they call it a Phrase, because it needs "going" plus the infinitive to make a Phrase taking about the future. There is still the Future conjugation. But, I think using "going + a verb infinitive" is easier to learn.
What shall we eat (my answer) seems to me another way of asking the same question.
Because it's a part of the tense. Using a conjugated form of present tense "ir" + a + the infinitive (ar, er, ir) form of the verb.
You have to learn this: ir+a+infinivo Yo voy a comer It is used like going to
Because "we" are not cannibals, if that is the correct term for eating ourselves! Or would masochist be more correct? :)