One possible application is the home office used by sales personnel who need access to the network at the headquarters via a VPN connection. The company pays for the VPN connection, the employee in the home office pays for Internet access privately.
To ensure a clean separation of the data links, two Internet connections are established, one to each provider. In the IP routing table, the default route is assigned to the private provider; the network with the headquarters via the VPN connection is routed over the headquarters' provider.