The Pension is not a benefit. Be careful about falling into the trap and loosely using the term benefit on social media particularly Twitter. Such mistakes take a life of their own and their ghosts haunt us and subsequent generations.
Even in Civil govt services, a percentage of your Pay had to be compulsorily put into Provident Fund and the (6% I think it was) used to be matched by the Organisation. The balance that you put used to be called contributory which wasn't matched but Principal + Interest used to be tax free on retirement.
With a few tweaks, this was modified. For Civilian govt employees who used to retire at 58 (and thereafter 60), the mathematics worked out particularly since mortality was lower. In Armed Forces, if the Govt matches the 6% contribution for anyone, it roughly will work out to the same as Pension particularly if someone retires at 60. In Armed Forces, where soldiers retire at 15 years or 20 years, Compounding doesn't kick in to make it a viable amount.
Hence, soldiers and by extension Armed Forces have not been put compulsory into the Scheme.
In Short - Pension is not a benefit but a necessity.