Should my MacBook Pro be running on Ventura, Catalina or Mojave? Why?

The oldest 14-in MacBook Pro is a 2021 model. It can run no version of MacOS OLDER than the version it was released with, which is a version of MacOS 12 Monterey.

Apple makes improvements at every iteration of software. Security Updates are only provided (by convention) for the current version and the two previous Major versions. Running an older version might subject you to security vulnerabilities.

Some users hold back a little from doing updates the instant a new release is available. This is a hedge against the possibility that the new update introduces a problem. In general, you should run the most recent version of MacOS possible.

You should use the most recent version of macOS compatible with your computer unless you have an app or some hardware which no longer works with the latest supported OS and you are unwilling to replace that app or hardware. This way you will have the most recent security updates when they become available and you will be able to use the most recent versions of third party apps since some developers will only support the operating systems which Apple is currently still supporting.