Imagine if you'd invented the world's first modern sink, in a world that had never seen a faucet or a plughole before. And you're training some new guys on the details of what you're working on.

Sure, some of that training is going to involve blackboard calculations and careful measurements.

But you're also probably going to demonstrate a sink to them and say "As you can see, when we turn this knob more hot water is added to the mix. Note how, after I put the plug in the plughole, the water level starts rising."

The purpose of the demo isn't to precisely measure the depth of the water or the temperature at the faucet or the angle the tap is turned to. It's just to let them see the thing in practice, so as they study it in theory they know what to imagine and how the model maps onto the real world.