Are they useful, and are they necessary?
The stickers on your mouthpiece ensure that your teeth stay in place easier. This is especially nice when you start playing sax; the mouthpiece feels quite hard on your teeth. You’re busy with all the new experiences, and then it’s nice that you don’t have to put in a lot of effort to keep your teeth in place. While breathing, your teeth will stay in the right position easier while you shortly lower your jaw. Most sax players keep on using them; it feels good.
The palm key risers raise the high d, e and f keys. It depends on the shape of your hands and how your sax is build, if they are of use for you. They can make it easier for your hands to find those top flaps, especially the high d key that we use quite a lot. On many sax types, the high d lies deep, so you have to bend your entire hand to reach for it.
You don’t have to put those caps on all three high valves, either. Try out for which valves they really are useful; they shouldn’t hinder you. If you have small hands or short fingers, they probably won’t be of use because, with that key riser, you could easily touch and open them accidentally.