Protein has been associated with building our muscles. Resistance exercise tears up muscles, requiring protein to be rebuilt by the body. Protein supplement companies promote consuming their products post-workouts, but a majority of the consumers find it difficult to tell if there is any real effect.
A 2014 study found that protein supplements have zero impact on lean mass and muscle strength in the first few weeks of resistance training. Protein supplements are a marketing strategy for us to buy protein which we can adequately get (in a better way) in normal food.