What do you list in your requirements.txt? If it's just the top level dependencies, you could have versions change of your transitive dependencies which can matter and break things. If it's all dependencies, you lose the context of what you need because you use it and what you need just because it's a dependency of a dependency.

Some projects used to have two files for this, effectively managing a lockfile manually with pip freeze, but then it's nice to have a wrapper around this pattern and that's where the first gen like pip-tools came from, and then stuff like poetry/pipenv is aiming to streamline that (and avoid manual use of companions like pyenv for specific Python versons) even more.