I'm doing a bath remodel and will install a pressure balance/rough-in valve. The valve can be purchased with or without stops. What are the stops used for? Are stops really necessary?

These are stop valves you can access from the front after removing the trim plate. They are used to turn off the water so you can service the valve. You need them if you have no other place to install stop valves and cannot turn off the main supply. These are most often found in apartments and the like. In a private residence you can turn off the water at the main if necessary but not in multi-family dwellings.