For something like a sale of a rental or something more complex, I get a much better idea of whats would happen if I make a "what-if" copy of the return.
Also there's no state tax planner. I would always just do VA in a different tax planner column and override the tax calculation but we have an easy state tax system that mostly follows federal. If you're doing CA or NJ (for example), they might not even agree with federal on what constitutes taxable wages. So I can see a use for a separate file. Just depends on the circumstances.
If you scoot down the list of forms on the left side of your screen, you will find it towards the end of the list. Or you could try typing "tax planner" into the green "where do I enter" button.
The "what if" copy Lisa was talking about was going the route you originally asked about ------ making a copy of the return.