This was recognized as far back as the USMC Small Wars Manual (which documented experiences from the inter-war years) and even the Army's operations in the Philippeans around the turn of the century. It was later validated by the CAP program launched by the Marines in Vietnam.

Local intel, fed to the people on the ground by locals who have lived around them and grown to trust them, is vital to any operation like this. When Intel has to flow up the chain, and then somehow make its way back down the chain, information is lost. And if troops aren't out among those they're supposed to protect, it creates an impression that the people either can't be protected or aren't worth protecting.