When to use relay in 555 monostable circuit?

Hello everyone,
These days I am working on a project in which I have to design a circuit board to activate 4 pneumatic valves. I have made the circuit on circuit wizard software and simulation works fine for it. So I was looking at some circuit diagrams and some of them had relays in it. Just wondering when do we use relays in a circuit? Also the project i am working on is something like this there are 4 pneumatic valves connected to the airbags in a car and as someone presses a button, airbag goes off instantly. Just like that you press another button second airbag goes off. For the programming of valves I have to use four 555 monostable circuit. So can anyone also tell whether relay would be required here or not? (I want airbags go off instantly as the button is pushed down).
Thank you
Waiting for replies!