Now that we know the question is about the GenericServlet class, let's slide it back to the servlet forum. The question is still not entirely clear, though: where did you get the notion of the ServletConfig object being transient or not being transient?
If I understand correctly, Servlets are serialisable and for example can be deployed to multiple nodes in a cluster. The config is specific to a node in the cluster though, so this prevents the config being moved to another node when it should not be, and allows the next node to set its own configuration.