The test network is definitely to test functions and stability, simulating real usage scenarios.
Once the main network is online, it involves real tokens and user assets, and the cost of fixing errors is very high, and it may even be impossible to roll back.
The test network usually uses "test coins", which will not affect the real capital ecology even if problems occur. This "trial and error" mechanism can minimize the risks of the main network.
In addition, the test network rewards can increase the popularity of the project, attract the attention of developers and the community, or investment institutions, etc., and form a word-of-mouth brand.