Many IP networks will operate very well with static routes. A LAN with a single gateway will be happy with one default route pointing to the gateway.
On the other end, some networks interconnect with many simpler networks, and so have many gateways (routers). Unlike simpler networks, where a single default route is enough for all machines (except maybe the gateway), all computers on those complex networks will need a method to find their way through all the available gateways. Enter dynamic routing and routed.