One inherent benefit to manufacturers by adopting WRC regulations was removing the need to mass-produce road-going versions of the cars that they competed with, under the previous rules for homologation. This meant that vehicles such as the Escort RS Cosworth and Subaru Impreza Turbo no longer had to be mass-produced for general sale in order to compete at World Championship level, and thus acting as a means of attracting increased competition and involvement by manufacturers.
Manufacturers gained a benefit by adopting WRC regulations: they didn't need to mass-produce road versions of their competitors' cars to qualify for the World Rally Championship. This allowed vehicles like the Escort RS Cosworth and Subaru Impreza Turbo to compete without needing to be widely available, leading to more competition and manufacturer involvement in the WRC.