Most of the answers to this why will be received while discussing what are IPRs. But there are other dimensions as well which cannot be contemplated in that framework. The answers to this why are more specific.
The pivotal reason for having IPRs is promotion of 'intellectual capital'. But this term I mean the collective intelligence and skill of mankind. I know that this is a very subjective generalization of concepts which evade defining but I would like to stick to the word. This is for a reason. The reason is that countries around the world are fascinated with development and growth. These terms of general and specific meaning have been dominating the creation and prosperity of a global order, known as the international institutions. Thus the creation of human capital and betterment of human kind are the basic and long term goals of all international institutions, be it the UN, WTO, IMF, IBRD or any other.
The promotion of human endeavor and skill is just one off-shoot of this greater realization for mankind. And therefore we have this concept of IPRs, which are intended to entice people in developing ideas into realizable shelves, making creativity creep the boundaries and making a new social order. This enticing is through rewards and benefits. Rewards both of monetary and non-monetary significances. Monetary because provisions are made in the legal systems to exploit the outcomes of their talents and creativity by selling them, leasing them or making use of them by whatever way or kind they may desire. Non-monetary in the sense of national and international recognition for the work they carried by a systematic organization and arrangement for such recognition (something like the Patent-Cooperation Treaty) etc.
IPRs are a series of rights designed to fit into and promote this stream of thought. They advance the legal protection to the authors and creators of inventions, discoveries, works etc., and also offer them state assistance from preventing others from misusing their works or exploiting them to their disadvantage. Thus IPRs provide a legal protection for trade secrets, industrial design, know-how, etc. On a similar standing, it provides exclusionary rights such that the author or creator of the property can enjoy the benefits of the rights by disallowing others from reaping the fruits of his labour. Patents, Copyright, TradeMarks, etc. are rights of this category, which differ only with respect to the nature and aspect of creativity involved but as far as their basis is concerned, stand on the same footing. Like Patent is used for an invention and thus is related primarily with technological innovation, whereas copyright is more related to arts, literary expressions, etc. Trademark on the other hand is a tool to keep free-riders away from sharing the fruits of your goodwill and effort which has made your brand distinct from the rest.
So we have IPRs, both to promote as well as to disassociate; promote intellectual contributions to the collective wisdom of mankind and disassociate scientific temper from acts of plagiarism from the rest of man-kind. The message simply and shortly is, don't borrow or steam, create your own.