if ssh is allowed for root user by default can anyone tell me the purpose of adding this to an 'out-of-the-box' config?
set system services ssh root-login allow
Default behavior of junos box is to allow users through SSH as root users.
There is nothing extra that following command does than above mentioned default behavior of Junos box.
'set system services ssh allow'.
Following link describes the same:
No special purpose other than configuration clarity for admins.