Ans. The key limitation of the SSP architecture was that it was configured by using a set of services, and all Web applications associated with the SSP bore the overhead of all the services even if they weren't being used. To change the service configuration for a particular Web application, a new SSP would have to be created.
The service application architecture on the other hand, allows a set of services to be associated with a given Web application and a different set of services to be associated with another Web application. Also, the same service application can be configured differently in different Web applications; therefore, Web sites can be configured to use only the services that are needed, rather than the entire bank of services.