If you were injured in a car accident then you are, naturally, entitled to recover the cost of repair to the vehicle, or its value if it has been written-off, if the accident was not your fault. Your car insurance company will usually deal with this part of the claim on your behalf.
However what do you do while the insurers argue about whether it should be repaired, by whom and when, and then wait for the repairs to be done?
You are entitled to go out and hire another car, similar to the one that you drove previously (ie same engine size and specification), and providing you can establish that this was a reasonable course of action to take, then the cost of hiring the car can form part of your claim for damages.