Any time you are involved with any sort of vehicular sale or transfer, you will need to provide a bill of sales form, especially if you are donating your car to charity. It actually makes more sense in many instances, because you can take off more from your taxes than you will be allowed as a trade.
A bill of sale is required unless it's a direct gift to a member of your immediate family. For instance, if you are “gifting” your 1973 Honda Civic to your daughter so she can take it to college, then the laws allow you to hand-write out a sale form that shows you are gifting the car to your child. Also, there's usually what is called a title fee of $1 paid for the sale.
If, on the other hand, you are gifting your car to one of the many charities out there, then you have to provide a bill of sale, as well as the normal vehicle title, so the transfer can be made legally.
Indeed, any time a vehicle is being gifted to anyone other than someone in your direct family, you have to go through all of the motions of getting the car appraised. Finding the title, putting together a bill of sale that also declares the appraised price and then, you also have to fill out the usual mileage statement and most likely some form of either waiver or acknowledgement for the state's Department of Motor Vehicles that shows the price. You actually need this form if you intend to use this as a deduction from your taxes. It just makes sense.