Difference between yarn/npm5 lockfiles and exact p

My simple question is: why can't I just use exact versions in my package.json? How is this different from a lockfile?

The main difference is that lockfiles also lock nested dependencies - all of the dependencies of your dependencies, and so on. Managing and tracking all of those changes can be incredibly difficult, and the number of packages that are used can grow exponentially.

There are also situations where you cannot manually specify that a particular version of a package should be used - consider 2 libraries that specify foo at ~1.0.0 and ~2.0.0 respectively. The difference in major version tells us that the API of foo@v1 is not going to match the API of foo@v2, so there's no way you could override the package version at your app level without causing conflicts and failures.

Finally, you might wonder "why have semver at all then? Why not just have all packages manually specify the exact version of their dependencies?" One of the main advantages of semver is it means you don't have to update every dependency in the tree whenever a sub-dependency updates. If I rely on foo, and foo relies on bar, and bar just had a critical bug that was patched, and we're using exact versions for everything, then foo must also be updated before I can get the fix. If foo and bar have different maintainers, or if foo is abandoned, that could take a while and I may need to fork the project (something I've done more than once in Java-land).

This is very useful for maintaining ecosystems of libraries because it fundamentally reduces the amount of maintenance work required per-node in the dependency tree, making it easier to extract libraries and patterns. I once had an early project where we were building a component library that used exact versions, and any time the core library containing shared functionality was updated, we had to submit a PR to each of the other packages to update the version, and sometimes followup PRs to components that depended on those. Needless to say, we consolidated the packages after a few months.

Hope that helps!