More specifically, herd immunity is necessary for people who cannot receive the vaccine to be protected. These types of folks are often the immunocompromised.

Herd immunity kicks in when the proportion of folks with sufficient antibodies is high enough, formally modeled as H >= 1 - 1/R_0, where H is the herd immunity level and R_0 is the average rate of cases caused per each case. R_0 (or more appropriately R_t) is dependent both on the intrinsic attributes of the virus as well as behavior of the organisms the virus infects.

So ensuring a large population with strong, effective prevention is in the national interest, is in the evolutionary interest based on all available science, and is in the interests of vulnerable populations.