If you use a long hose setup for your regulators (like I do), never.
With a standard regulator configuration, totally up to you. I would say it's better to have one when you're starting out in diving and building confidence in the water. You can swim face-down without needing to use your tank supply for example. It's also useful when it's a little choppy on the surface to get air from a little higher up than your face which might get splashed often. Later, you can weigh the pros and cons of whether you want to keep the snorkel or not.
At higher levels of diving (tech, wreck, cave) you'll find that it is considered an entanglement hazard and forbidden in training. The long hose setup normally used in these types of diving can catch the snorkel when you share gas with your buddy and dislodge your mask. The location of the snorkel by your chin can also hinder deploying your necklace regulator.
At these levels, you're not typically going to swim long distances on the surface face down, but rather on your back, so a snorkel isn't much use and may actually dig into the water and give you a mouthful when you don't want it.
I also keep my regulator in my mouth at all times in the water, so I'm ready to take a clear breath of air with no risk of inhaling water accidentally from a snorkel.
Agencies like PADI require recreational instructors and students to have a snorkel during training, so if you see your instructor with one, it might not be their personal choice.
