Your body is constantly making and breaking down tissue throughout the body. Cells die, are replaced, dead stuff and remnants cleared out. When you exercise, you pull local energy sources away from the things being used in that exercise, as well as from the body's global energy sources. Preferred resources for muscle-using efforts come from glycogen. After the local glycogen is depleted, you get a little bit of the same stuff provided from elsewhere in the body (liver) but not nearly enough to really maintain the activity, so local tissue becomes a resource as well (in currently not-well-understood amounts).
As your body is constantly wearing down and making new, and you create an additional demand that wears down even more, you need to replenish the raw materials while also providing enough energy to keep the rest of the body running, and you need enough rest time for repairs to go faster than they would be if you're not resting.
Look at endurance cardio efforts in this context and you can see why endurance athletes rarely have anything approaching muscular forms: they can't eat enough to both support the cardio effort, and support full and complete recovery and maybe adaptation, before they do the cardio again.
So yeah. Eat your protein. It's pretty damned important.
And eat enough food otherwise. Also pretty damned important.
Terracore
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