In CISCO routers, the ip classless command has to be enabled when using classful Routing Protocols; and you need the router to use the default route when it receives a packet destined towards an unknown subnet of a network, whose some subnets are known to the router.
For example, consider a Router A, which knows the routes to subnets 192.168.0.8/29 and 192.168.0.16/29 and the default route is set. If this router receives a packet destined to an address in the 192.168.0.24/29 subnet and the ip classless command is not enabled, the router then discards the packet without using the default route. This is because, classful routing protocols assume that if they know some of the subnets of a network, it should know all the existing subnets of that network!
Enabling ip classless tells the router that it should follow the best supernet route or the default route for unknown
subnets of known networks, and for unknown networks.