Hi Faisal,
Suppose you have different workflows for New and Change account and you don't want a new user to submit a request with change account type then you can restrict him by using this settings.
In 5.3 there was no system line item and the application was validating fine against role line items
But in 10 if you do not select the system line item then the validation does not take place.
Hope you got some idea now.
Regards,
Neeraj