Silverlight Application class provides several services that commonly required by Silverlight application, so when you create a Silverlight you must create a class that is being derived from Silverlight Application class. Application class represents Entry point for your Silverlight application.
Silverlight Plug-in Lifecycle
Silverlight plug-in lifecycle starts with opening a web page that host a Silverlight plug-in. If Silverlight plug-in is not present then web page ask you to install Silverlight browser plug-in. Then web browser activates the Silverlight plug-in and start downloading application package. Silverlight plug-in load Silverlight core services, followed by Silverlight CLR (Common Language Runtime) which creates application domain for your application. After downloading application package CLR creates object of Application class and raise Application Startup event. Application constructor can also be used to initialize application task but generally its a good practice to use Application Startup event handler for most application initialization task.
There can be only one Application instance in a Silverlight-based application. After startup, the Application instance provides several services that applications commonly require. You can access these services from any code in your application by retrieving the Application instance from the static Application.Current property.