Testnet is a trial. If you see testnet, then you are in test mode. This illustrates the mainnet mode will also work according to the ongoing testnet mode. This is not much different from the beta.
If you create an app, before officially launching it to the public, then you must launch the beta version of the app as a feasibility test.
Likewise on the Ethereum blockchain. Before the official version of the token is released to the public, a testnet version will be developed as a trial.
Exactly, every app released testnet version for blockchain network testing and even project team will reward anyone who use testnet feature will be given token/coin reward, some testnet even worth more than $100+ for each user and but we have to make sure to join the program testnet of popular projects.
I think the one you are talking above are called testnet airdrops. I have tried participating on some of them in the past and usually those who conduct it are newly released projects and since it's an airdrop, obviously they will gave away some of their tokens.
If you are lucky that you join a good project then it's possible to get 100 dollars or more on them but if not then you can also get zero which happens to me a lot of times. Older projects like ethereum do rarely have an airdrops like this because I think they are already stable and rarely make any changes. They do also have the funds to hire the best developers to test their networks.