What is the purpose behind granting ownership rights in intellectual property?

Like other forms of tangible property, the rights associated with intellectual property incentivize individual productivity. The exclusive right to use or control property also incentivizes creativity. This belief is captured in Article 1, Section 8 of the US Constitution, which grants Congress the power to promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries. In furtherance of this purpose, the Federal Government created and extensive statutory and regulatory framework for the recognition and enforcement of intellectual property rights.

Note: Generally, intellectual property rights (particularly patents) grant a limited monopoly to the holder in exchange for full disclosure of the intellectual property rights to the public. This is not true for trade secrets, which require secrecy.