Charcoal canister- how does it work

[gclark]

I have a 72 with 350 cid Quadrajet. It has the original evap canister mounted next to the battery.

The canister has a valve on top with 3 hose connections. It also has a filter on the bottom.

The three hose connections are:
Canister to fuel tank
Canister to PCV and carburetor manifold vacuum
Canister to carburetor ported vacuum

Questions: what is the function of the valve on top of the canister? Why is the ported vacuum line necessary?

Thanks in advance.