Imagine that you depend on library A of a particular version which itself depends on library B. With minimal version selection, as long you don't bump you dependency on library A (or some other library that depends on library B), you'll continue to get that same version of library B. But, then library B releases a critical security fix. With minimal version selection, there isn't a great way to pick up that fix. You can _hope_ that library A releases a new version that requires the fix - but that may or may not happen and could take a while. Or, you could add an explicit dependency on the new version of library B - which is unfortunate, since, your main package doesn't depend on library B directly.
Lock files solve this problem. You can depend on whatever version of library A that you need and lock the transitive dependencies. And once library B releases its fix, you can update your lock file without having up bump the version of library A.
Tools like poetry providing the features to automate this workflow.
the version selected during a build is the one with the most minimal version that satisfies all other constraints. this means if you have libA that needs dep>=1.1 and libB that needs dep>=1.3, you get dep=1.3 even if dep1.9 is out. your build never changes because of a new version release, as long as they release with proper semantic versioning. if you later include libC that needs dep>=1.8, you'll get that version. but because you changed your immediate dependencies, not due to a surprise down the dependency line.
https://research.swtch.com/vgo-principles