The rationale for intellectual property law has changed over time. The topic became highly politicised a decade or two ago with rarely unconfused debate.
When institutional lobbyists, eg for the film or music industries, speak or act in favour of their clients' IP monopoly rights others respond often like automatons. On all sides fairy stories, banality or irrelevance often abound. It is typically a political game with plackards not substance.
Part of the pro-IP side is the World Intellectual Property Organization, a United Nations agency based in Geneva. WIPO administers 24 international treaties, including for trade marks, patents, and phonograms. WIPO is led by an Australian, Dr Francis Gurry.
IP Australia has uploaded to YouTube the above video originally titled "Five Minutes with Dr Francis Gurry". Dr Gurry expresses a few reason for why intellectual property law exists. The transcript is here. These reasons are often heard from pro-IP voices or lobby groups, he states them with interesting examples.
For me the most persuasive remains that within the capitalist mode of production intellectual property law's justification for its existence is that it is an incentive for investment. Dr Gurry refers to this reason, and so do we on this website in 46 articles where these keywords appear - "intellectual property" and "investment". People create, I believe, because that's what human beings do. When they invest they seek the certainty that IP law can provide.
Further reading: 41 start-up stories from an IP business lawyer