The theory is that, by allowing them to profit by sale of the creation, it encourages creation -- particularly creation of things that are unique and suitable for sale (which is, again in theory, a proxy for "useful"), thereby, without or without separate "disclosure" of the form present in patents, promotes progress in science and the useful arts.

The idea that attaching that making something property that someone can profit from (whether for a limited time or not) was a means of promoting development of that class of thing was fairly prominent at the time the Constitution was written, and extremely influential on the understanding of all kinds of property (not just "intellectual property") rights and their purpose in society around the time the Constitution was being written. (Its even more central in modern libertarian thought.)

Note that I think that disclosure is important, and that for IP the restriction to limited terms (and actual, real, substantive limits, such as the Congress seems to have abandoned for copyrights) are practically important for best promoting progress in science and the useful arts -- increasing the scope of the public domain is, I believe, in practice very important to achieving the Constitutional purpose of IP. But increasing the public domain is itself not the explicit Constitutional purpose, its just an important means to that purpose.