If I don't study at all ever, only then is my understanding pure and not contaminated by studying for a test?

If I dont study for the test, then you basically measure how long ago it was when I was revising the material last time (if it was last week I get much better result then if it was month and half ago) and you have even more random in terms of whether my learning coincided with I focused on what would count as "exploitation" of this test or different test. For example, was you focusing on speed during easy exercises or ability to solve hard problems last three weeks? It is pretty much random choice and preparation lets you get back to other mode relatively fast - while without it you get worst results with kids who accidentally did wrong thing and it has nothing to do with understanding of material, just a random chance.

Motivation to actually prepare yourself removes that random.

The exploitation of specific test, especially if you dont understand base material, gets you only so far compared to that.