When you purchase a $100 campaign, you essentially pre-paid for advertising credits.

Let me illustrate with an 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 required prior to using the service.