Fundamentally, so it can't be taken down or pwned.
I haven't heard any convincing yes answers to all of these questions outside of financial instruments.
But there is no general answer. It depends on the circumstances.
One fairly strong legitimate use case is people who are consistently shunned by credit card networks. Lots of people working in adult industries, weed (in states where it's legal), certain high risk spaces... though _even then_ it's not about decentralization, just abut needing some form of competition (debit card network?)
Meanwhile there are real costs to the decentralized networks
Suffice to say that this would likely be very against a bunch of KYC money laundering laws, but it's a decent trick
Cash is not decentralized. Governments can print as much as they want.
The fact that gov't can print money has roughly the same effect as speculation has on cryptocurrencies
I find it telling that places where the government is bullish on cryptocurrencies (Venezuela, Moscow) have reputations as places with high amounts of graft.