Understanding schemes

Imagine a tool that, with a couple of mouse clicks, or even a keyboard shortcut, will change the display name of an app from Flappy Mushroom Test to Flappy Mushroom, change the app's Bundle ID, swap one set of server endpoints for another, and to cap it off, change your application region from system region to Iceland.

That tool, in Xcode, is scheme selection. If you haven't used schemes before, we're talking about this thing:

It's the SalesTaxCalc bit, in this example. If you click on it with a new project, you'll see this:

But before we accept the invitation to create a New Scheme..., or edit the one we have already, let's see why they're of interest to us.