Well I think this is a science fiction.
If they even decide to do it, they'll just over leverage supply of bitcoin because they will NOT buy spot btc.
What goverments and banks are doing, they add some kind of something like ETF, virtual and they claim it's usd, or shares or crypto but it's empty binary code without backing.
So, in any utopian scenario where a country would buy 1000 btc and pay some pensions, that would be nice but it's not realistic.

Benefits?
Well, bank or goverment cannot freeze your funds
The main benefit is freedom with potential of wealth increase if price goes up.