At 3.2km (2 miles) up the horizon is 202km away. At 12m (40ft) for a ground based radar (pulled that out of my arse, just for an example) the horizon is at 12.4km.
Then we discuss things like the Clutter Zone, which is a big issue for ground based radar systems. Planes and helicopters can fly Nap-of-the-Earth - below about 500ft is the usual stated height - because everything from buildings to wind over 15mph can make it unreliable for ground based systems.
An elevated system eliminates most of these problems, and this system has a radar horizon that covers all water and land from DC out to the Atlantic coast.
There's a lot of areas where a submarine could launch from unnoticed, and I'm guessing trying to detect a sub in busy waters is a lot harder and less reliable than a radar system to target a missile launch and try to take down a short range ICBM.