Our patent system allows an inventor to patent an improvement to another's patented invention. Ideally, that should (1) encourage early inventors to continue to invest in improving their inventions before someone else does and (2) encourage new inventors to improve on existing, still-patented inventions by giving them ways to profit from their improvements.

There are very few places in law where you can legally stake a claim within someone else's territory and exploit it for profit.

To be clear, that anyone can improve upon anyone else’s invention and claim their improvement as their own, to the exclusion of the prior inventor, is what drives innovation.