They say the center compartment of the card wallet is deep, but what and how am I supposed to put things in it?

Because card slots are usually cut shallow to match the height of a card, inserting banknotes causes the top part to stick out and the corners to bend. For this card wallet from Low Tide, one slot in the center is set deeper than the rest, allowing cards and banknotes to be put in the same compartment^[1].

Because it has depth, even if you fold banknotes in half and insert them upright, the top edges do not rise above the outside of the wallet. On days when you only need to carry a few cards and a bit of cash, it is a structure that eliminates the need to carry an extra separate wallet.

A keyring is attached here, but rather than being a fixed decoration, it can be detached and attached^[2]. You can hang keys together with it or hook it to a bag loop, and on days when you want to carry just the slim wallet, you can remove it. The rest of the structure is a basic card wallet with nothing extra added^[3].

hmmm, what actually changes in this wallet is not the number of compartments, but the depth of a single compartment.