Afghanistan is landlocked and It depends on Pakistan for most things, food, oil, and other goods. The western forces there also have their food and oil coming through Pakistan, you might remember when American Apache attacked a Paki post and killed 27 prokies, the Pakis shut their supply. Although the weapons and ammunition is usually flown in using cargo planes, the troops cannot survive without essentials like oil, food, medical supplies etc. So the land route is very crucial.
Pakis do the same thing to the Afghans, once there even a small attack they shut down their Torkham and Chaman borders shutting all supplies to Afghanistan and then Afghans start starving. So their Army and intelligence have to stop their operations inside Pakistan via proxies like the TTP. Which why once Afghan are no longer dependent on Pakistan for food, oil and other supplies for their people, they will no longer control their proxies in Pakistan and totally unleash them.
So with Iran route open all non-lethal goods like oil, food, medical and other supplies can go to Afghanistan and also the western Army, they can simply buy from what Afghan import so in a way Afghan also benefit without Iran raising an eyebrow. And the whole economy of the western belt of Pakistan which totally depend on exporting goods to Afghanistan, the farmers of Punjab, floor mill owners of Khyber and Quetta, Truck drivers, Mechanics, hotel, dhaba owners and other people linked to this trade would go Jobless, i.e. more recruits for mulla factory and TTP etc.
