Many servers create their own user. PostgreSQL creates postgres and gives ownership of configuration and data files to it. It's especially convenient when those

Many servers create their own user. PostgreSQL creates postgres and gives ownership of configuration and data files to it. It's especially convenient when those server softwares run on real servers or on multiuser machines. Historically single user Unix machines were the exception. They cost too much. Graphical workstations started the trend, then Linux.