A formula which references parameters will return values according to the parameter values. To reference a parameter in a formula, follow the syntax: @ParameterName.
Here is an example of a formula which displays the URL for a company. The URL is based on the value for the parameter Company.
"http://www." + @p_Company + ".com"
Here, you can either manually type @p_Company, or you can select it from the Parameters node in the Fields panel (if you choose this option, the @ sign will automatically be inserted).
This is a simple example of referencing a parameter in a formula. Here, the formula is placed directly into a report. You will better understand the benefits of referencing parameters in formulas which are used to control object properties.