What is a "testnet" and who is it for?
Software is typically written and tested by developers on their local computers. Once it's working, the software is deployed to servers or other infrastructure for general public use. For simple/early projects, that's really all you need, but mature systems with customers need a little more complexity.
At minimum, there needs to be a testing environment. Servers that closely match the final public setup (which is typically called "production"). The test environment is where developers and business stakeholders validate the app before releasing it to the public.
Blockchains have a similar arrangement. Developers test smart contracts locally, then deploy to "testnet" and finally "mainnet" where users will actually interact with the dApp.
The purpose of testnet is to validate your dApps without impacting users and without wasting actual network resources. On blockchains with gas fees, this is especially important. You wouldn't want to spend your hard-earned cryptocurrency to upload and test your code. The testing process is usually repetitive, so costs could easily get out of hand.
On Koinos or any sufficiently inexpensive gas-powered blockchain, this isn't a concern, but using testnet for development is still preferable. It's best to save network resources for "real" usage. Regardless, public blockchains are permissionless, so if you want to test on mainnet, no one will stop you.
Smart contract blockchains are built for developers first, so a certain amount of technical ability is expected of network users. Unfortunately, most chains seem to extend that expectation to dApp users. If users want to try out a dApp without spending real money on gas-powered chains, they would need to use the testnet version of the dApp.
Sponsoring other wallets allows users to try out dApps without putting anything at risk on mainnet. On Koinos, this looks like creating a wallet, authorizing it to use the mana from your main wallet, and then using the dApp with the new wallet. This won't cost you any cryptocurrency, and can be built as an automatic feature of your wallet software.
Safe and free, just like testnet. Real results on mainnet.
P.S. The power of this approach is that safety and security becomes just as easy as the default.