I just put my running-out items on the entryway table.

Me too! If I must take out some forgettable item, it sits blocking the door.

Blocking the door is my go-to way of making sure I don't forget something when leaving the house.

Exactly how (and why) I try buy things. Not exactly "bulk" buying but more "at least 2" buying.

This is a much simpler way. Now only to remember to do it next time I get groceries.

yes, having a shopping list is good, but have to remember to put it on the shopping list, so same logic applies.

i.e. when taking out last tube, you have to remember to do it them, but you are about to brush your teeth, so probably not going to do it then, will get to it afterwards, but you're a forgetful person, so you forget, and now you've forgotten its the last tube.

But with the sentinel tube, every time you use it you will remember its the sentinel and you'll be like "did I add it the shopping list?" Eventually one time you'll remember to check the storage / shopping list to see if it needs to be added.

TLDR: the point is for forgetful people who can't do the action required right away.