Purpose of a surveillance state?

The purpose of the surveillance state is to preserve the existence of the surveillance state.

Ultimately, it's about self-preservation and control. Control over citizens, corporations, the economy, and so on.

States are likely to share its surveillance with the big corporations (who are much more powerful than elected politicians) to ensure that any union activists never ever gets a job again. It's a much better tool to maintain the status quo with than physical force. There is at least two dozen famous political philosophers who you can read for all the details.