That's part of the BackupPC documentation:
BackupPC User

    It is best if BackupPC runs as a special user, eg backuppc, that has 
limited privileges. It is preferred that backuppc belongs to a system 
administrator group so that sys admin members can browse BackupPC files, edit 
the configuration files and so on. Although configurable, the default settings 
leave group read permission on pool files, so make sure the BackupPC user's 
group is chosen restrictively.

    On this installation, this is backuppc.

    For security purposes you might choose to configure the BackupPC user with 
the shell set to /bin/false. Since you might need to run some BackupPC programs 
as the BackupPC user for testing purposes, you can use the -s option to su to 
explicitly run a shell, eg:

        su -s /bin/bash backuppc

    Depending upon your configuration you might also need the -l option.


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