Intellectual property owners enjoy certain exclusive rights to use and exploit their protected assets. For example, they can exclude others from using their creations or prevent other companies from using their logo or brand (trade mark).
However, there are often benefits from allowing others to exercise all or some of those rights. For example, a licence agreement enables an IP owner to authorise another party to exercise some of the IP owner's rights in the IP while retaining ownership and control of the IP.
The licensor will be able to:
generate more revenue
expand its business geographically or open new product lines
settle issues around infringement claims
sell a business or its assets, while still retaining the IP for itself
This is different from an assignment which is an outright transfer of an owner's rights, title and interests.