Thank you for understanding me. Testnet is the first step of testing for mainnet. So how does the new coin need a mainnet?
I didn't get your question.
The coin needs a mainnet to exist.
Testnet is just for tests, nothing is really happening. It's not even a beta version.
After the mainnet is launched, developers still use testnet to test their applications. Even third party developers use it to test their new application or new versions.
Let's suppose a new Electrum version is being developed. They test it on the bitcoin testnet. If everything goes fine they release it on the mainnet