They say this so you don't go to the store and become overwhelmed by all the things that WERE NOT on your list and break the budget.
IMPULSE ITEMS ARE EMOTIONAL CHOICES
Making the list doesn't help if you don't stick to it.
It's very similar to success quitting smoking.
You have to
STICK TO YOUR PLAN
KEEP YOUR IMPULSES OUT OF IT!
I stuck to my plan. :-) and I got a LOT of good stuff.