By beating the game you obtain two new game modes (Nightmare and Arrange). Enemies deal 99% of your HP per hit, so you need to avoid getting hit. Nightmare has less supplies than normal while Arrange has excess. On Arrange, enemies are 2-3x more durable, so those supplies can be very easily squandered if you aren’t careful.
By beating Onslaught mode within a time limit, you unlock a knife that can insta-kill any enemy.