These species are vital to maintaining ecosystems. Trophic cascade is the concept of all species in an ecosystem affecting one another.
When wolves were reintroduced to Yellowstone National Park they were able control the populations of the elk. The elk would eventually stop congregating in open areas where they could be tracked more easily and vegetation started to rabidly grow in these areas of the Park which then attracted various species of birds and insects, increasing their populations. The wolves also killed coyotes and because of that, the populations of rodents and smaller hooved animals began to increase which increased the population of birds of prey and smaller carnivorous mammals like badgers. The regenerated forests also began changing the physical geography of the park, preventing erosion and minimizing flooding along rivers.
In British Columbia when the salmon return to the rivers and streams to lay and fertilize eggs, some of these salmon are killed by bears. Of course the salmon feeds the bears, but when the bears finish eating all the meat they want off the carcasses they discard them in the forests. These salmon corpses decompose and help fertilize new trees which interacts with all areas of life within the forests.
People are starting to learn this and various rewilding projects have started to pop up around the world. In Europe various organizations are trying to reintroduce species like wild horses, bison, "wild" cattle, water buffalo and moose in an effort to revitalize and renew the various wild landscapes and of course with herbivores comes wolves, lynx and bear.