I only put mine in the car when we are off-roading, or in the Snow. I just use it to lift a wheel and stick stuff under it to get it on something solid... much prefer a trolley jack for normal lifting.
As Makeitfit says, they are not very safe, and you would not want to work on a car, or even change a tyre with it up on one, as they move way to much, and do not take a lot to knock them off of centre.
You can also use them as a puller, so with a strap, you can use it to move the car, even if it is very slow, and only 3'6" at a time:nenau
Do make sure you read up on them, and keep the mechanism clean and lubricated, there have been some nasty accidents with them.