The benefit potentially is that the amount of time they calculate you can "buy back" in pensionable time will count towards your 25YOS for a pension. This could turn into a shorter amount of Reg Force time served/earlier retirement.
However, if you elect to buy back and do so over 'monthly installments', be aware that there is interest tacked on to the amount, etc. If you decide to elect to buy back, do some math and figure out if the payment option is worth it. :2c: