Do this simple excersize.
Next time you go to the grocery store write down your list of items.
Then track how long it takes you to get in and out of the store, how much you spent and what you actually purchased. On the following trip to the grocery store don't write the items down. Try amd remember all of the items your are looking for. Now track how long it took you to get in and out of the store, what you spent and what you purchased.
Most likely your result will show that on the day you did not write anything down, you spent more time at the store, spent more money, purchased items not originally on your list and probably forgot something that was on your list.