You might do it to have disjoint networks for different sets of computers. E.g. if you want to allow visitors to use your Internet connection with their laptops, but not give them access to the LAN with your PCs on it.
However, this can also be done by connecting the routers in series, rather than parallel. Connect the visitor router to the modem, and connect the WAN port of the private router to one of the visitor router's LAN ports. But this complicates things; if you need port forwarding, you have to configure it on both routers (visitor router forwards to private router, which forwards to the PC).
