Essential for a standard basic oral exam. Using one will not only allow you to examine and diagnose more pathology, but will allow you to show and explain issues to owners. 
A good mirror is the next best thing to an oroscope in many procedures such as extractions, instrument placement and periodontal treatments.
Also useful for image recording on your phone (see attached pics and videos)
Stainless steel Dental Mirror designed and shaped for optimal viewing of all intra-oral structures including occlusal surfaces, periodontal pockets, and diastemata. 
18.5 inch (47 cm) long stainless-steel dental mirror with convenient "T" handle. 
The 10-degree tilted end piece contains a 2-inch (5 cm) diameter mirror. 
Replacement mirrors are also available.
Comes with a 5-year warranty on steel component.