You don't always need a bill of sale to transfer title of a vehicle or anything else that has a title.
However it's highly recommended that you do have a bill of sale as it can be necessary in certain situations, especially for private sales which can prove ownership and for taxation purposes as well.
In most places though you only need to fill out the back of the title and turn it over to the person when you sale the vehicle or other thing that has a title.
No bill of sale is needed in most cases and the title will do just fine.
A bill of sale serves as evidence of the sale and the ownership transfer which is crucial for registering the vehicle and transferring the title although some states don't always require it.
Some states such as Arkansas require a bill of sale when there's no more space on the title for the seller to sign or when the seller applies for a vehicle sales tax credit.
A bill of sale is still a good thing to have for private vehicle sellers even if it's not required as it helps to document the agreement of the sale of the vehicle between the seller and buyer.
The bill of sale also ensures that both parties understand the terms of the sale and helps to prevent any fraudulent activities and provides a record for taxation and record keeping.