Are they necessary?? No…
Generally speaking, if you have a well-balanced diet which is adequate in protein there is really no need to have protein shakes in your diet. It really depends on the person but 30-40% of your diet should be made up of protein rich foods. Chicken, red meat, eggs, fish, milk are all great sources of protein
However, the problem is that many people do not get enough protein in their diet. It is often hard to get in a serve of protein at every meal. This is why I choose to add a protein powder to my daily diet. For anyone who does any strength training or really any training at all, adequate protein is required to build / preserve muscle depending on your goal.
A serve of protein straight after you train helps to maximise your bodies recovery. If you are someone who can get straight home to eat a protein rich meal that is great! Personally, I just find it more convenient to have a protein shake.