Patents are used to balance the right of the individual with the concept of the public good. In general, the purpose of a patent is two-fold: to allow the inventor of some new thing to retain exclusive rights as to its use for a limited period of time, and after that time to ensure that the details of the invention are available for public use.
By granting the patent-holder exclusive rights to their invention, use and duplication of the invention is at the sole discretion of the inventor, potentially allowing them to profit from it. However, after a period of time those exclusive rights cease to apply, and the new invention becomes available to the public. This contributes to the public good by increasing the amount of general knowledge available.