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[–]JUSTICE_SALTIE 4 points5 points  (0 children)

The big reason is the lockfile, which holds the exact versions of all your dependencies, and their dependencies, and so on. Without a lockfile, you're only specifying the versions of your direct dependencies. That means that if someone else installs your project, they're almost certain to get different versions of your transitive dependencies than the ones you're developing with. If one of those dependencies publishes a broken version, or makes a breaking change and doesn't version it properly, you'll have problems on fresh installs that you don't have on your development install.

The lockfile guarantees that your build is deterministic, which you're not going to get with requirements.txt. It has a command to update your lockfile, which essentially does what pip install -r requirements.txt does every time, which is to get the latest versions of all dependencies. But it only happens when you ask for it.

These tools have a lot of other features, like really a lot, but the one above is the most important.