To promote the useful arts and sciences…
Copyright is supposed to be a balanced deal struck between society and creative people, allowing them some limited exclusive rights to their ideas and expressions in return for the work they put into them.
Unfortunately, businesses like Disney have lobbied many times and turned that simple idea into an unlimited, unrestricted monopoly on ideas, music, movies, etc.
So what is the optimum length of a copyright? Turns out the founding fathers had it right: 14 years.