What is coming with a car, going with a car, there is no purpose in the car, but the car can not do without it?
2nd Attempt (Probably wrong but worth a shot)
Is the answer
Sound, specifically the static noises that come with running a car?
What is coming with a car,
Sounds that the engine makes, the alternator, etc.
going with a car,
When a car goes by, it makes the vroom sound.
there is no purpose in the car,
The sounds a car makes (besides the horn) serve no function, but they do sound cool.
but the car can not do without it?
While this answer fits the first three pretty well, the last line, I have a problem believing. Electric cars don't really make noise, do they? And saying a car can't do without noise would be a stretch IMHO, except for the horn, which in that case, would serve a purpose. This is why I like my first answer better.
The answer of course is
A human being.
What is coming with a car
Humans have to purchase a car.
Going with a car
Are sold to humans
There is no purpose in the car
But the car can not do without it
The car can't go anywhere without a human (sans google auto cars).
Speed (or: 'motion')
Speed (motion) comes and goes with the car, and the car cannot do anything without speed.
Also, when a human is inside the car, they don't need to have speed for themselves, they can just sit back and relax, because the car is doing the work for them. So there is no purpose of having speed when you're inside the car.