Prompts are useful for giving dynamic values.this gives user more flexibility.

that is values at the run time.

as soon as a prompt is created ?PromptName? will be created.

there are lot of prompts like:

value prompt

date prompt

date and time prompt

generated prompt

select&search prompt.

even though prompt can be used in report as well as in prompt page.

its better to place yours in the later.

will be interesting once u start exploring the properties like static choices, default selections and others.

Have Fun!!!