What a great question. This question has been asked for thousands of years in different forms. One advantage of a volunteer army may be that service may be based on love for country. There is power in patriotism. When you give people the freedom to choose, there may be a level of commitment not otherwise found. As early as the Greeks and Romans, they believed and strongly argued for a citizen army. They knew that this was much superior to any mercenary force. And the best case scenario was when people volunteered to defend and fight for their country on their own.
Based on this logic, when there is mandatory enlistment as there is in many countries even today, such as South Korea, the level of commitment may be lesser. It may be viewed only as something that must be done.