It is really important when you are setting your monthly budget to think through what you know you will actually spend. It is not helpful to say, "I'm not going to eat out at all this month" when you know you will have a time or two (at least) that you don't have time to cook and need to hit up the drive through or take out.
You lose two primary things when you do that:
It makes you feel like you are constantly failing and letting yourself down when you have set goals that you fail to meet. Over time you will stop trusting yourself to follow through and you inevitably lose steam.
2) Strategic Edge
If that $50 you spend on food is accounted for, then you can shave off that amount from another area of your budget. If you have the room to add it without shaving off elsewhere, it gives you the ability to predict a more accurate amount for your disposable (or left over) income so that you can proactively use it to give, save, invest, pay debt, and so on.