When you beat a level with the "Unlimited Ammo" cheat, it gives the player the maximum ammo amount for the next one, which is sometimes unobtainable through legitimate means. Most of the non-starting levels can max out ammo for the next one anyway, and the ones who don't, wouldn't really give an advantage (such as having 21 crossbow on mission 4). But it's good to keep it in mind in situations such as 8 grenades for 5-2, and limited SMG ammo for 3-2 (which is still more bullets than necessary for any speedrun). Just don't use more than what is possible.