What use would the time travelling aliens who engineered us have for grain?
That's like asking what use would people with advanced electrical technology have for oil. Grain is a raw resource. It stores energy, so it's a very basic form of bio-solar energy. You turn it into other stuff. I don't know about the "time travelling" part, but coming up with a scenario involving aliens in general isn't that hard.
e.g. imagine if a survey ship of some aliens got to Earth, and assume they
don't have FTL travel. They're not popping in, having lunch and popping out, because that wouldn't make much sense. They'd have evolved/engineered themselves to either live for millennia or just be immortal. And they'd have far more advanced "planet scanners" than we have, so they'd already know that there's a planet teeming with life here to go look at. Such a survey mission to a system might take 1000s of years, but it's not a big deal to them, since anyone that far advanced will clearly have the tech to live as long as they want. And if a small ship full of alien scientists, came
all the way here, it's
not so that they can "scan" the planet from orbit in their little ship: a big enough sensor array in another system can get you most of that kind of data anyway. They'd
come to this system only because then they can go down to the planet's surface and start taking detailed samples, and stay long enough to do scientific research. e.g. 20 alien scientists trying to unlock all Earth's unique scientific value would take a
long time since they're basically just one lab, even if they're really smart. But they have the time to satisfy their curiosity.
Now, you'd have to imagine the circumstances in which such a race would interact with the local sentient apes. e.g. say the ship took damage in the atmosphere and needs repairs (e.g. even with high-tech, a stray meteorite in orbit can wreck you), but the parts to do some don't exist here and to "phone home" would take centuries. So they work out that if they tech-up the local apes just enough they can have them producing alloys that are needed. e.g. they might have assumed that once they left and stopped propping up the ape-civilization it would just collapse back into the stone-age, and not actually develop into a high-tech civilization. Or even, the ship wasn't damaged, but their expedition is to spend 1000s of years here but might not carry 1000s of years worth of food on the ship, because it has e.g. suspended animation or other tech for the interstellar part of the journey, so they do in fact have finite food stores, and teching-up the apes just enough to make useful resources was intended to allow the aliens to divert more effort towards research rather than resource acquisition.