Being alive, you will naturally keep on building (and intermittently losing) muscles throughout your life as a biological process.

If you workout, you will develop relatively more muscles than what a sedentary person would, and lose muscles relatively lesser.

Muscles need building blocks in the form of protein, and fuel in the form of carbs, and fats. Till the time your natural diet meets your protein requirements, you will have sufficient building blocks for muscular development and maintenance.

Now, depending upon how much muscles do you want to build and ?maintain?, you will need to alter your diet. If your diet can meet your altered protein requirements due to more aggressive goals around muscle mass, well and good. If not, because you can eat whole foods only as much, people tend to add protein supplements for additional support.

Humans cannot add muscle mass beyond a particular point, and that slightly varies amongst people. There is a natural check in a human body through a protein called Myostatin, which moderates muscular growth in humans. Hence, you cannot expect to endlessly gain muscles, and it will stop for good at some point. irrespective of whether you take protein food or supplements.

There is no going beyond the natural limit of muscle mass without resorting to taking drugs like steroids / pro-hormones, which temporarily Inhibit Myostatin and promote unnatural muscular hypertrophy by elevating hormonal levels. This is where protein supplements really come handy, for the nutritional requirements suddenly go up manifolds when you are on the drugs because whatever protein you eat is rapidly taken up in the body and utilized for building muscles (normally not all protein consumed gets utilized in body), and people generally cannot eat big loads of natural food required for the protein requirements.

In a nutshell, you do not require to take any protein supplements to build muscles within your own genetic limits. You would only require protein rich natural whole food as part of a muscle building diet, which should meet the protein requirements calculated for your own goals based on your body. If you find that consuming the amount of whole food as per your goal is very discomforting, maybe then you could consider another source of condensed protein, which could be another natural food source or worst case a supplement.

Note: if you are a Vegetarian or Vegan, dense natural protein sources would be harder to find (like chicken/meat) and that is why you will see most vegetarian/vegan people consuming more protein supplements than the non-vegetarian counterparts.