[EvanAnderson]

In the US Constitution, at least, the purpose of limited-time monopolies is "To promote the progress of science and useful arts...", not to ensure that creators of works are able to profit. Granting limited-time monopolies is a way to create profit for the creators, but as I read it, the goal was to promote progress for all of society, not for the creators.

[chrismcb]

In this case it is to promote useful art. One way to do that is to allow the creators to profit off they work. It gives them the incentive to create, plus it gives them the ability to create.

[EvanAnderson]

The growth of the public domain is a method to promote the useful arts. Particularly in the realm of patents. The eventual guarantee that patented inventions become public knowledge means that more innovation is prevented from remaining in the realm of trade secrets forever.