There's nuclear deterrence and there's conventional deterrence. ICBMs and SLBMs are very good at nuclear deterrence, but that's it. B-21 (and B2) can help with

There's nuclear deterrence and there's conventional deterrence. ICBMs and SLBMs are very good at nuclear deterrence, but that's it. B-21 (and B2) can help with the nuclear deterrence, but only marginally. But B-21 can help a lot with the conventional deterrence. You can think of them as an alternative to aircraft carriers. Carriers carry F-18s and F-35s, with a range of about 700 miles. B2's have a range of about 7000 miles (and most likely B-21s will match that). Which means aircraft carriers need to get fairly close to China to deter. Well inside the range of China's carrier killer missiles. B-21 can just go 1000 miles away from China and patrol there for many hours. Maybe not now, but in a few years, they'll have loyal wing-man drones with them. A squadron of B-21s and their associated drones will probably match or exceed the entire bombload of the air complement of an aircraft carrier.