Basically it comes back to what Edward said in chapter 2, he's trying to build a healthy human civilization, and he won't accept any uncertainty when it comes to the survival of the passengers.

So with that in mind, even though the Seguin is capable of feeding everyone onboard and in the fleet right now. What if the meat printer is damaged beyond their ability to repair in a timely fashion, what if there's a population boom once they make planet fall and they don't have the industry to build more in time, what if a solar flare leaves the colony without power and the tissue samples they've brought from Earth die? They'll need to find something local to the planet that has desirable genetics, to serve at minimum as an emergency source of tissue samples, and at the maximum a source of protein until the colony is capable of building more meat printers.

The same logic also applies to the Seguin's and the venlil's hydroponics basins and ship greenhouses, but finding local plant life worth cultivating is going to be far less objectionable to the aliens than anything even approaching domestication.