It is not unforeseeable that in the future a hall may refuse entrance to a company that does not hold liability insurance. A professional has a plan in place in the event of a major equipment failure. Ask if your prospective disc jockey carries back-up equipment to every event. Make sure that they have extra amplifiers, a mixing board and speakers. Ask if these issues are resolved internally or if they have a deal with an outside company to back them up.
Internal assistance is always preferable.
Their outside back-up may have problems of their own or be busy with another event. A professional will guarantee certain things. For example, they can offer many varieties of formalwear that in some cases will even match your wedding party. They can offer a wide variety of music that should be agreed upon in advance. They offer a written performance contract that should clearly state all the details of your event.