why do some vac conversions use a vac canister (reserve)?

[BW Jones]

i've noticed that some conversions to vacuum brakes use a vacuum canister. what is the purpose of this canister and why don't all conversion kits use it? all input is appreciated.

bw jones

 

[WSLN 6]

I don't know why you wouldn't use one.....helps hold a bit more vacuum in reserve after you are running the car under boost.