"-(으)ㄹ 거" is "a thing to do" and 있다 says there is one: is there anything to eat, if there's anything to prep do it the night before, if there's anything to put in put it in, there was something to correct, wasn't there? It is the frame for asking or noting whether some task or item exists, and the verb in front tells you which — 먹을 거 something to eat, 찍을 거 something to photograph, 담을 거 something to pack.