By implementing IN in GSM Networks you can create various services as mentined by Mr.Pankaj and you can creat some new and Charging can be done differently.
More are less IN-Helps you to utilise the network resources effectively.Where there are some IN Like systems but they require lot of resources and creation of new services will be hard,more complex.
Nowadays for IN become like a common standard with common Application Protocal – CAMEL which allows you to provide the custom made applications for your Roaming Subscribers.
Prepaid Roaming another result.
Rgds
Scot