Computer programs are better at some things than people.
Humans should not do jobs that are dangerous, error-prone, boring or simply replaceable by an algorithm.
Why because humans are lousy at them, and more importantly boredom leads to mistakes, ill-humor, even mental health issue.
Remember PacMan… We may Play Pacman but we win only if the 
game is designed to let us win sometimes. Otherwise, the Pacman would always win. It never gets tired, never makes a mistake and it is much, much faster than we can ever be. It is simply better at it. But a virtual human can never write this paragraph. It can spell check it, grammar check it and even translate it.