Bit rot, "it works on my machine"-style issues, cache misses on dependency installation (which can really bloat deploy times in deploy pipelines by busting Docker caches across machines, too). Can be a security issue if a vulnerable library version is pushed and one installs it as a consequence of having non-locked dependencies, especially in python where package install scripts have a lot of power.

Lock files help solve for these. You can build software without solving them, but it makes my life easier.