Please explain why is renal hypertension so dangerous?

Causes kidney failur: If untreated over the long-term, it is so dangerous because it causes renal failure. Patients with renal failure may need dialysis.

Kidney failure: Generally speaking, renal hypertension results in stiffening of the arteries within the kidney, and a subsequent decrease in overall blood flows the kidneys try to protect themselves from this high pressure. Over the course of time, this results in gradual progressive kidney damage up until the point where eventually, the kidneys fail entirely and dialysis or transplant is required.