Why does the first convict take the blame for taking the food in Great Expectations?

Answer 1

He escapes and is already recaptured. He knows that he's going to be stuck for a long time anyway, and it doesn't make a whole lot of difference in his sentencing whether he stole the food or not. Mostly, though, he is grateful to Pip for bringing him the food, and he doesn't want Pip to get in trouble for doing it. In short: he's a good person.

Answer 2

The first convict, Magwitch, takes the blame for stealing the food to protect Pip, whom he met on the marshes when Pip was a boy. Magwitch feels a sense of responsibility towards Pip and wants to repay the kindness Pip showed him by providing him with food and a file when he was an escaped convict. By taking the blame for the theft, he hopes to shield Pip from any repercussions.