Sometimes, Patrick forgot how to eat. Or rather, "forgot" was not the best way to put it. It was a bunch of things lumped into one word. Like not being able to open, make, or get food. Or not remembering how to make all the proper mouth and arm movements eating required. Or not having the energy.
But he didn't have the words or explanations for all that. So "forgot" it was. It was simpler that way.
This was one of the many (many, many, many) reasons SpongeBob was his Best Friend Forever. SpongeBob didn't need the explanations to care, or to want to help.
So when SpongeBob needed to, he would blend some food, get the funnel, and help Patrick eat.