So, why do we need to use Bitcoin???
to buy a new car 
LOL maybe this is absurd reason but this is my reason just wanna buy car
That's foolish to think of using bitcoin to buy a car. In future users might get a access to buy car using bitcoin, but as the above mate indicated using bitcoin to buy car is not possible. Better is to make a investment into bitcoin from the day job earning, which would help get a car with bitcoin in future as we can buy only digital goods and other few services for bitcoin.
The heart of the matter is not whether you can buy a car directly with bitcoins. It is more important what the car maker is going to do with these bitcoins later. If he simply sells them for fiat on the market, how is that essentially different if you yourself sold bitcoins for the same fiat before you buy a car? On the other hand, if they spend at least a small part of these bitcoins directly (e.g. for paying wages to some of their employees), that would be a different story...
Perhaps, a little step toward an economic system based entirely on Bitcoin