The simple reason is money. A good photo assistant will make you vastly more productive on a shoot saving you time and making you more money. They allow you to do more and deliver more value to your client. Modern photoshoots can be complex. As the photographer you must oversee all the creative details. Your photo assistant will manage all the details of your camera, lighting and any other equipment you may need. Photo assistants do add costs to a shoot, but they provide value many multiples of their day-rate in time-saving and productivity for you and your client.