If you delete the NT Authority\System account to improve security - and then find the services are using it to log on - this will add it back:

-- Grant [NT Authority\System] a login to SQL Server:
EXEC sp_grantlogin [NT Authority\System]
-- Add that account to the sysadmins role:
EXEC sp_addsrvrolemember @loginame = [NT Authority\System]
, @rolename =  'sysadmin'