When you complete a purchase for a $100 campaign, you bought $100 worth of
pre-paid advertising credits.
Let's illustrate with this example:
Have you ever purchased pre-paid service for a cell phone, such as Tracfone?
In order to use their network, you must pay for minutes before service is activated. The pre-paid minutes are deducted from your budget as you use the service throughout the month.
Any minutes unused are not lost as they “roll over” as credits towards the next billing cycle.
Just as you cannot try Tracfone for “free”, you cannot test Adwords for “free” because a budget is a pre-requisite to using the service.