When you do something voluntarily, you do it of your own volition, or will. Doing something willingly, or because you agree to it, is doing something of your own volition.
Volition comes from Latin and French roots meaning "wish" or "will." Legal speech and writing often include the word volition, as a way to affirm that a person involved in a crime acted "on their own volition," or consented to be part of the crime. Maybe criminals don't wish to go to jail, but their acts often show their volition to break the law.