There is a larger bag and a smaller one, so if you only have a few things to organize for a trip, you won't need to carry something around with a lot of wasted space.
It's a bit like playing a game of Tetris with your belongings. It can be difficult to find a good layout for your items, so you may want to take a picture if you find one that works well so you don't forget how you did it.
The whole case is closed with a zipper, which means that your items will be kept safe in the case even if they slip out of the elastics/pockets.
This also means you can leave items loose in the case if you run out of room.
When zippers are located at the top, it can be a hassle to get the cord zippered in without it trying to pop back out.
The diagonal zipper addresses this issue.
For those worried about weight, especially backpackers, these mesh bags are a great solution. They weigh 2.4oz for 4, and you can decide how many you want to carry.
There are two pouches that are quite loose and have no zippers. They are almost unusable as items don't stay in them, especially when the pouch is rolled/unrolled.