Why do patents expire? How are they renewed?
Patents are effectively a social contract. In exchange for you revealing your invention to the public so they can replicate it, you get a time-limited monopoly on that invention.
Renewals are there to ensure that monopolies are not left in place if they are not commercially required. If the invention is commercially worthwhile, then it is worth renewing.
There are other more sceptical views on it too, but I will not go into those.