Like writing your own emulator, writing a full TCP stack is a project that is intrinsically worth doing.

You can probably also come up with real-world applications for it (as a security tester, there are lots of applications for having full control over a TPC stack, regardless of how performant it is), but just having done it offers a huge learning return on a modest investment.

You probably don't fully grok what TCP even is until you've made congestion control work.