it just holds vacuum to provide a reserve of vacuum (built up during decell or from an external vac pump ) to the vac brake booster to improve the apply pressure to your brakes
if you have poor brakes with vacuum brake setup because of low vacuum add the canister , if that doesnt help connect a vac pump to the canister , or go manual brakes
if you have a powermaster then you you dont need a vac reserve canister
.. it cant affect boost because its connected with a check valve to prevent boost from getting into the canister as well as to hold the vac in the canister
it also cant help idle vacuum, if you have vac leaks fix them , if you dont have any vac leaks only a smaller cam can fix what youll see on a vac gauge