The top slot is simply a quick way of getting stuff out of and into the front flap. It does not give access to the main compartment. Imagine if you will, you have your bread and cheese wrapped in a linen handkerchief. It's in the main compartment along with your firelighting kit, tinder, pipe and tobacco. It's all in there neat and tidy and you are happily wandering on your journey. The weather is fine but there are clouds in the distance. You have a lightweight waterproof jacket but you don't need it at the moment. So you scrunch it up and stick it in the slot in the front flap. You also have your folded map in there.
It starts raining, so you just undo the front flap, pull out your waterproof without the need to open the main compartment and put it on. Then you can pull out your map the same way, check it and stick it back in again without risk of your bread, cheese,tobacco or tinder getting wet.
Well, that's it's traditional use, but in modern parlance it's basically the same. A knapsack with an extra easy to access compartment in the lid.
Eric