For further explanation, bundle pristine ignores unpacked files on disk. In other words, this command utilizes the local .gem cache or the gem's git repository as if one were installing from scratch.
Note: the Bundler gem cannot be restored to its original state with pristine. One also cannot use bundle pristine on gems with a 'path' option in the Gemfile, because bundler has no original copy it can restore from.
When is it practical to use bundle pristine?
It comes in handy when a developer is debugging a gem. bundle pristine is a great way to get rid of experimental changes to a gem that one may not want.
Why use bundle pristine over gem pristine --all?
Both commands are very similar. For context: bundle pristine, without arguments, cleans all gems from the lockfile. Meanwhile, gem pristine --all cleans all installed gems for that Ruby version.
If a developer forgets which gems in their project they might have been debugging, the Rubygems gem pristine [GEMNAME] command may be inconvenient. One can avoid waiting for gem pristine --all, and instead run bundle pristine.