You typically move parameters around in a method just for readability and maintenance. You might want the more important parameters to appear first on the method signature, or you might try to keep the order similar across like methods or overloads. You can see from Figure 9.22 that the Change Signature dialog allows you to move around the parameters using the up and down arrows (upper right).
As you change the parameter order, the resulting method signature is displayed below the parameter list (bottom of Figure 9.22). You also have the option to preview any changes that are made to callers of the method. Clicking the OK button applies the changes to both the method and its callers.