Why do we need to have wireless product certification

Theoretically:

The radio frequency is infinite (0 ~∞ Hz))

so how to manage the commonly used limited frequency?

This requires us to divide the frequency range, scope, and size of the launch.
 

Moreover, many countries are considering national security. Many of the frequencies in 0 ~ 40GHz are not open to civilian products. We must avoid using or accidentally occupying these frequencies so as to produce all kinds of unnecessary consequences.

For all sorts of considerations, all of us are required to conduct various tests on wireless products and obtain corresponding national certification.  ---ETL certification body