Backpacks are accessories that don't cost 300 usd to make or 500k to buy and are a way of transporting more drugs for big deals. I don't really remember how the system was but I think you could store big guns like shotguns and rifles in there without them showing up on your back, which could be useful as well. You have normal backpacks with small storage and larger backpacks, maybe duffle bags, with more storage. Although having more things like bricks, barrels, or guns has its drawbacks. The more items (depending on the weight) you put in there, the slower your character becomes, stamina may burn faster as well.
No idea why they were removed but they should be added back.