Why is the booster better than PAYG?

Boosters offer you more value for money. Pay As You Go are the standard rates with no savings.

Getting a booster
How many boosters can I have at the same time?
When will my booster expire?
When I buy a booster which will I use first the remaining of my plan or the booster?
What is a “Booster” and how does it work?
How can I buy a booster?