If it’s not, then you will only receive emails and will only be able to send emails when the phone is connected to the Wifi. Which maybe is what you want.
There is, however, also the opposite issue. Sometimes, you may lose the internet, but the Wifi router is still working.
In this case, the iPhone will connect to the Wifi router, find no internet, and will not be able to access anything, including emails.
The right thing would be for the device to use cellular connectivity if there is no internet, but Apple computers don’t do this