Open mandates are often verbal agreements where a sole mandate is a written agreement. With no written agreement in place, certain aspects could be misinterpreted, which could cause conflict. A clear, written contract protects both you and the agent, reducing the risk of any misunderstandings.
A written contract will also ensure that the agent puts maximum effort into fulfilling the goals that have been set. If any agent is working on a open mandate, they may be less inclined to spend as much time marketing the property, which will reduce your home's chances of selling.