The Corps is made up of ordinary people of the Republic who want to do their bit to fight the Seps. Why should the clones get all the fun?
The Republic Volunteer Corps was an arm of the Republic Military formed during the Clone Wars to give citizens of the Galactic Republic the opportunity to fight against the Confederacy of Independent Systems. Formed ten months into the conflict as a result of lobbying by various militaristic pressure groups who had formed a political coalition under the leadership of Tharn Jagged. Jagged and his supporters wanted to join the clone troopers of the Grand Army of the Republic on the battlefield. The Corps put a heavy emphasis on consolidating the decade-long training of the clone troopers into just a few months, making its members both elite and available very shortly after recruitment, unlike the clones who each took ten years to become battle-ready soldiers.
The Corps saw action throughout the Clone Wars, operating under its own system of command rather than under the command of the Jedi who led the clones. In spite of this, many members of the Corps did acknowledge orders from Jedi. This, however, did not stop the Corps from participating, eagerly in the case of some of its more radical members, in the execution of Order 66 at the end of the Clone Wars and in the Great Jedi Purge which continued for years afterwards.