How did "migrating people to the real border" fundamentally solve the long-term border defense difficulties of the Central Plains Dynasty?

Historically, the main threat of the Central Plains Dynasty in border management came from powerful nomadic tribes in the northern and northwestern border areas. Nomadic people are good at riding and shooting, have strong mobility, come and go freely, have strong military advantages, and the harassment of border areas is extremely sudden. As Chao Cuo said, "Hu people's livelihood is out of reach, and it is easy to disturb the border." With the development of immigrants in the frontier, settled agricultural bases are gradually formed. Accordingly, water conservancy projects and road construction are carried out one after another, so that settlers can deal with the enemy for a long time, avoiding the simple military defense method of perfunctory at ordinary times and panicking in case of emergency, and organically combining frontier construction with frontier defense, which can effectively resist the harassment of nomadic people on the agricultural areas of the Central Plains and guarantee imperial politics.