The public domain allows free use of anything in it. It allows people to innovate and expand upon established knowledge without having to pay tribute to original creators or their estates.
Intellectual property was created as a compromise to get people to be more open with their works. It allowed artists and writers to create things without risking their creations copied and losing their livelihood. Similarly inventors were also encouraged to publish how they invented because it granted them a period of exclusive access to controlling that method. In the long term the public benefits by getting access to everything.
It's been successfully demonized by teachers unions over the years. While there are limitations, with the medium, it's far from useless. It can't fully assess mastery of a subject, but it can determine basic and applied understanding of one. It's a very simple step to expand from student competency to teacher competency.