It's not totally for testing but "to start Bitcoin network" since it need at least one miner to make transactions.
A bit off-topic but... those are the "block rewards" for the earliest blocks that allegedly mined by Satoshi (excluding genesis block),
so it's fair to assume that he had the private keys of those addresses so he can spend it whenever he wants.

To get this topic back on track, the coinbase transaction for those blocks have their public keys displayed.
And when you know the public key, you theoretically can brute-force the private key using a functional Magic Quantum Computer.

Ex.: Reward for block 10 - you can get the public key by getting the output script.
The hex inside "[]".