The benefit of pyenv seems clear to me - it's a simpler way to have multiple Python installs and manage which one is your "default" Python at a project, directory, user, system, and global level (they can even all be different).

Poetry...not so much. requirements.txt is good enough for me most of the time.

Poetry gives you exactly what pip and venv give you, with the two pretty much perfectly integrated, in a way that is pleasant to use and not something that you'll grudgingly migrate your project to after your fifth dependency.

Well, poetry gives you a lock file, separation between dev dependencies and runtime dependencies, and easy deployments to PyPI. If you're hacking a oneof-script you might not need these features, but for "real" projects Poetry is invaluable.