If you understand French, watch this: https://www.arte.tv/fr/videos/078777-000-A/siberie-les-avent... you'll be convinced.

If you don't: https://en.wikipedia.org/wiki/Pleistocene_Park

Basically we need them for an earth-scale experiment that will try to prevent the permafost from unfreezing, thanks to ruminant eating the grass through the snow, thus breaking the snow insulation that prevents the ground from re-freezing deep in winter. If the permafrost do unfreeze, I believe the climate is doomed.

edits: typos