I think it is wrong to call bitcoin a "private" currency.
On the contrary, imo bitcoin is much more of a public currency that any national currency is.
I think it is need to called "private currency" because even government do not access who's the members of bitcoin and who is the person who earning money on it.This is a great crypto because no one can access what you are doing in your bitcoin or who's the person sending your money. It is better than Gold because it's a digital and no one can steal it except when they got your wallet address.