This is not required for all devices. Only on certain phones and system, and it's due to an Android limitation on some OS systems.
Truecaller (and blocking apps in general) won't be able to block calls if you don't set it as your default dialer.
Please note that you can still use your system dialer as your default dialer as usual even with Truecaller being set as your default dialer. It won't change shortcuts, favorites or how you make calls, etc.
If you have any questions, please contact us and we'll get back to you asap