All the inputs to report_power tool are saved in the XDC constraints of the project and will be populated if report_power tool is invoked again in the flow. If the power constraints are provided after opening a design run, the user must explicitly save the constraints to the project, else the constraints are lost. This is useful for the what-if analysis. The most recent switching activity constraints are retained and appear in the tool. Even if you provide inputs through XDC based commands in Tcl console of Vivado® Integrated Design Environment or through the Net Properties window (Edit Properties in Power tab), these input values will reflect in report_power tool. XDC constraints for switching activity will be in sync with the report_power tool. Any change made in the tool will reflect in the XDC constraints and vice-versa.
This is also helpful, if you want to override the default switching activity in the Report Power. In this case, you can create XDC constraints with desired default values and run Report Power.