Yes .patch_storage directory basically facilitate to rollback the patches. During patch installation OPatch saves copies of all the files that were replaced by the new patch before the new versions of these files are loaded and stores it in $ORACLE_HOME/.patch_storage/patch ID/. These saved files are called rollback files and are the key to rollback the patch.
Oracle made this directory as hidden directory just to prevent from evil eye :)... Actually this directory is used internally by Oracle while applying the patch and rollback the patch and to cleanup this directory Oracle has provided opatch command opatch util cleanup.
You can also refer below link to know more about cleaning .patch_storage directory.
How To Avoid Disk Full Issues Because OPatch Backups Take Big Amount Of Disk Space. [ID 550522.1]