It might sound overly complicated, but it makes looting a much more active process. The DC3RX, by comparison, has more 1x2 slots so you can store more magazines, but instead of a single 1x4 slot, it has four 1x1 slots that actually restrict what you can carry a great deal.
The SOE Micro Rig, for example, has two 1x2 slots for your standard 30-round magazines, but it also has a 1x4 slot that can store a bunch of different shapes of items. Like in most survival shooters, a bigger backpack means more space for loot, but Tarkov does something cool with its tactical rigs where the pockets will have different shapes depending on the rig. Medicine, likewise, needs to be in your pockets or rig before you can bind it to a hotkey.
Spare magazines, for example, should always be stored in your tactical rig because you can't reload using one if it's in your bag. Each type of inventory has specific uses you have to be mindful of.