Governments grant patents to inventors as an incentive. A patent gives legal rights which may help the inventor find financial reward in invention. But, there is a trade-off; in a patent, the inventor describes the invention, allowing others to benefit from the information. These benefits include design solutions and inspiration for more innovation.
Mostly obviously, a patent document is sought for:
the details of invention revealed in the patent.
Patent literature also may be useful for:
Prior art (that is, existing patents and other literature) concerning a specific technology -- this is required when planning a patent application but also of interest when looking for design solutions and inspiration
Information about a company based on its patents
Information about a researcher based on the individual's patents
A sense of current invention found by exploring new patent applications in areas of interest.