Space blankets reduce the loss due to radiation and convection. They can provide a confined space that can be kept warm more easily (insulation is provided by the trapped air in this case). The space blanket does not provide significant heat insulation (putting it under the body doesn´t help too much for this reason). Unless the space blanket is punctured it will keep moisture away. There is better gear but it may not be feasible to have it with all the time.
In the case of the car kit it´s a better than nothing approach. The space in that box is very limited after all.
In conclusion I´d say that the space blanket can help to reduce heat loss when used properly. In many cases they may be all you have so learn how make most of them.