Hardest things to remember are the things that you don't know you have forgotten.
For example lets say you need to buy milk, sugar, coffee, bread, butter, cheese, ham and eggs from the store. You get there, pick the cart and start collecting your groceries. You get bread and eggs, head to the dairy aisle for milk and cheese and finally pick up sugar and coffee on your way to the counter.
You go through your list as you remember it: milk, suger, coffee, bread, cheese, ham and eggs. Ups, you almost forgot the ham! You go back for the ham, everything is good.
Back home you decide to reward yourself for remembering everything with a sandwich. But there is no butter! How could you forget? You even checked you got everything before heading out.
If you think you remember everything you don't even stop to thing if you have forgotten something. With lists of things to remember it's easy to remember almost the whole list.
Now lists have this nice property, they have length. You can count the items on your list. And single number is quite easy to remember. Just compare that number to the number of items you thought you remembered and you immediately notice you have forgotten something. If you know you are missing something at least you have a chance to try and recall it.