Why do you want to update your dependencies if they work? Isn't the whole point of dependency management to avoid using different versions of dependencies than the ones they have been tested on?

Security fixes, performance enhancements, new features. There are many reasons. But the point is you update in a controlled manner. You don't just push the latest version of everything out on to prod, but you also don't keep pushing the same version that worked a year ago.

You want to easily be able to update dependencies (and retest with them, of course) to ensure you’re getting security updates, for one.