Contract killings arent legal therefore they are not contracts. Besides this point. How is bitcoin a smart contract if I wanted hire a hitman?
I really don't know, don't care and don't want to know.
I only mentioned 'contract killing' to point that the word 'contract' can be used outside of definition of 'legally enforced agreements', whether you want to accept it or not.
The same goes with 'smart contracts', you can refuse to use word 'contract' and replace it with something else, but that doesn't change anything.
The fact that 'contracts' are tied to definition of 'legal agreements' is because up until now there was pretty much no possibility of resolving issues automatically.
You use paypal vs back to bank depending on the situation but you stay within the currency. Why make a conversion take exchange risk and convert it back? Just the exchange risk is not worth the trouble
You choose to use bitcoins instead of USD depending on the situation. For every man, or every purchase the reasons could be different.
Sure, if you don't have any bitcoins, there would make no economical sense to try to pay with bitcoins for a pair of socks. But if the merchant offers you discount for BTC payment (he can charge you less, because he doesn't have to include risk of stolen cc or unfair chargebacks in the price) then you could be better off buying BTC and paying with them. Pair of socks may not be worth it, but 10 pairs + some shirts and pants could give you enough incentive.
That's just one possible reason to use BTC.