Why aspirate for an im injection?

It is done to check whether the needle has hit any blood vessel. Aspiration is especially important when you are administering anesthesia, for example, during a dental procedure, all anesthesia are local, meaning only a small area of your body will get numb, and by injecting the drug into a blood vessel it will follow the blood stream into other parts of the body. With that said, it does matter what type of injection you are administering, if it is some sort of vaccine, it may not cause any complications even if it gets in your blood stream.