If you have a stale block, then it allows you to test things, while at the same time, it is not part of the heaviest chain, so it quickly disappears (and any assignable value is then lost, after a single confirmation).

Because blocks with minimal difficulty are accepted in such test network. And then, when a new mainnet block kicks in, then they become invalid.

You won't "acquire" anything permanently. For example: you can use my code now, and CPU-mine some blocks on top of testnet4. Then, some time later, all of your blocks disappear (unless you will win network propagation games). Imagine having a similar feature, but on top of mainnet, where all amounts and network conditions will automatically follow the real network, so you will start with 3.125 tBTC, instead of 50 tBTC. And then, you mine some new coins, you test some new things, and then, a new mainnet block arrives, and the whole test chain is automatically destroyed, to prevent any trading.