Because mapping.
Think of it this way:
You wake up in the morning and get dressed. You put on your favorite pair of jeans and then decide to put your stuff in your pockets. You have your wallet, keys, Rubik's Cube, pet rock and phone.
"just memoing" is when you decide to put all the objects into your front right pocket. This would work if you just had one or two objects, or if you just had to keep track of the stuff for a short amount of time, but with five objects to put into one pocket, over the course of the day they will inevitably get mixed up and you can't grab whatever item you want based on location.
Using a journey:
You have the bright idea to put your items in different pockets.
You put your wallet in your back left pocket.
You put your pet rock in your back right pocket.
You put your phone in your front right pocket.
You put your keys in your front left pocket.
You put your Rubik's Cube in your hand because you need to practice.
Now when you want something, you merely have to recall what pocket it is in, and if you continue to use the same system every day, you will get more and more familiar with it.
I know this analogy isn't perfect, but in the same way, when you want to access for example, the corner memo of your fourth cube, instead of fishing around in one collection of jumbled images, you simply imagine the corner location in your fourth room, and you will find the image that you placed there.
Hope this helps you understand