Transferring large amounts of money overseas for large purchases is cheaper, faster and safer than using the current banking system. So that's at least one legal use case Bitcoin has improved for me. Even after exchange fees and miners fees, I paid less in fees then I would have using a bank and the transaction confirmed in under an hour vs several days or more through banks.
1. You don't buy a car on a credit card. Dealerships won't even accept cards. Private individuals won't, either.
2. Your counterparty (the dealership) will sell your data, regardless of whether you pay with cheque, cash, bitcoin, gold coins, or lost pirate treasure.
No. Darknet markets also sell things that are unambiguously legal to buy everywhere, as well as products that are legal in some countries but not others. The products on sale are far from only the stereotypical drugs porn warez and fraud (and no you cannot buy assassination on darknet markets; any such offers are a mix of law enforcement stings and scams which take place without any marketplace providing escrow).
If nowhere but darknet markets let you buy a car with Bitcoin or Monero, then only darknet markets are what someone who has cryptocurrency and wants to buy a car with it will use. That being said, you can exchange cryptocurrency for a car safely and legally, without touching a DNM or doing anything shady.
I understand using a darknet to buy heroin and assassinations, but why on earth would I want to use a darknet market to buy a car? To pay my rent?