Hello Manas,
If one script would start different systems, then it makes sense to set "service/admin_users".
Setting "service/protectedwebmethods = OSExecute" is basically disabling the authentication requirement to any method except "OSExecute".
This means that any user would be able to call the method "Stop", for example, which could lead to a production down situation (if the involved system is a productive system 
).
Setting "service/protectedwebmethods" can be used as emergency workarounds (e.g., during upgrades). But it should usually not be set.
Cheers!
Isaías