For people already on AD coming in from a SA or other AD, it provides seamless continuity of benefits - especially untaxed BAH in the DC area, Commissary and Exchange benefits, and no-cost TRICARE healthcare. And - if you have a family or plan to start one, that continuity of benefits is extra important.
We had a USNA sponsor son get into several fine medical schools, but he elected to go to USUHS, as he was getting married in August after graduation, and they planned to start their family within a year. He was able to immediately start his TSP (govt 401K) with his earned income as an O-1, got his single then with-dependents BAH housing allowance at DC-area rate, had full medical and dental coverage for wife and the 2 kids they had while he was at USUHS. He loved the access to senior military doctors and their career mentoring, opportunity to get time on USNS COMFORT, exposure to Johns Hopkins training opportunities and came over to help at Induction Day at USNA. He was fine with the longer obligation, wanted to be a career military doc (and still is).
The current with-dependents BAH monthly rate, an untaxed allowance, in the DC area is $2706, rising to $3312 at the 4- year point when they make O-3.