You can turn it off if you would like, follow
this tutorial and select Manual instead of Automatic.
PostgreSQL by itself doesn't collect any data, it's just a database server. The only data that PT3 puts into it is related to poker hands. When PostgreSQL is installed it automatically starts itself on reboot; PostgreSQL needs to be running in order for PT3 to import data, and originally PT3 couldn't start up PostgreSQL every time so having PostgreSQL running on startup was a good thing.
PostgreSQL should not present any security issue. By default it is configured so that nobody from any other computer can connect to the database. The user profile is created to add an extra level of security, in fact, so that the postgres user can't access other areas of the computer, just the PostgreSQL server.