Submissions.db and submissions.db.bak

I was informed that I could delete performance.db if necessary and it will just be recreated the file without a problem.  I have done this to periodically reset the performance stats.

 

However, I noticed (in the same folder) that submissions.db has created a db.bak of the submission.db file. 

  1. Why was a backup created - it is 3.5M+ file and just a copy of the submission.db?

   2. Will more submissions backups be created as time goes on?

   3. Can I delete the submissions.db (bak) and what is it purpose of both files?