Performing music is not a piece of cake, it takes years of practice, efforts, and way too much energy to cheer the crowd, and sometimes people simply forget how much of hard work it is required to get on stage and perform in front of thousands of people. Musicians/singers have to maintain a constant amount of energy to keep their show going, as a tired musician might just get booed off the stage.
The energy level put by the musicians into the show often leaves them very drained after the show is over after all even they are humans. Here are some before and after pictures of musicians which will help you to understand how tiredness takes a toll on them, just like everybody else. The left ones have been taken before their performance and the right ones taken right after the show.
Have a look yourself and be the judge here:
Davey Muise, the lead singer of the band Vanna often has to put in a lot of energy as the band usually performs metalcore, and post-hardcore which takes way too much of energy. The amount of energy a metal band lead vocalist puts into a performance is much higher than many other genres.