The U.S. Census Bureau collects tons of data unrelated to the decennial counting for Congressional apportionment.

https://www.census.gov/programs-surveys.html

The American Community Survey is the most well-known, as it replaced the “long form” sampling that had been an extension to the Census.

Unless you’re a government explicitly and openly aligned with Christian nationalists.

The point might be going over my head… why does it make no sense?

It actually does. Religious affinity can absolutely be useful for longer trend studies, and census data is usually of much, much higher quality than other random sample studies.