It is not a translation of "to eat" but a construction for a Spanish
near future: ir (conjugated according to person)+ a + main verb (in infinitive)
"to be going to do something" (is often used as a translation since the construction resembles) but you can also use simple future "will do something"
That would suggest they do not habitually eat out (e.g. not at restaurants nor even potentially at friends' houses). It could also be part of a dramatic narrative. "They do not go out to eat, nor even to buy food. Indeed, they do not leave their house for any reason. They simply draw their blinds and wait for Armageddon."
If I understand correctly, van comer could be interpreted as meaning they/you-all are going eating or they/you-all are going to eat.
While van a comer locks it down to the latter interpretation.
It sounds goofy with comer, but van nadar sounds normal when translated.
Someone correct me if I'm wrong.
Theoretically, if we are translating the sentence word for word. But that's not low language works. In Spanish, the preposition "a" is required in order to create the tense properly. Prepositions are tricky: keep in mind that they don't match up perfectly in any two languages. If you're still stuck, remember that you can always click the "Tips & Notes" at the top of each sentence!