People asked me if they should enable it or not. I had no answer for them as I did not know what it means or does.
Could not find a discussion or ticket; rPHEX9f897dbcb9cbebea42ad716633da3bcf83e315de provides no reasons.
Then found https://www.mediawiki.org/wiki/Phabricator/Help#Search_terms where it is documented.
After enabling "Expert Mode" on https://phabricator.wikimedia.org/settings/panel/developer/ , the advanced Search interface at https://phabricator.wikimedia.org/maniphest/query/advanced/ exposes five additional fields: Parent IDs, Subtask IDs, IDs, Columns, PHIDs (see screenshots below).
[acko@fedora phabricator]$ grep -r "expert-mode" . ./extensions/src/expert/ExpertModeSetting.php: const SETTINGKEY = 'developer.expert-mode'; ./deployment/libext/misc/src/expert/ExpertModeSetting.php: const SETTINGKEY = 'developer.expert-mode'; [acko@fedora phabricator]$ grep -r "ExpertMode" . ./extensions/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'src/expert/ExpertModeSetting.php', ./extensions/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'PhabricatorSelectSetting', ./deployment/translations/projects/phabricator_ext/core/qqq.json: "0697a4a521e77f96": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$33 ExpertModeSetting.php:33]", ./deployment/translations/projects/phabricator_ext/core/qqq.json: "9818855a7a1ea556": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$24 ExpertModeSetting.php:24]", ./deployment/translations/projects/phabricator_ext/core/qqq.json: "aa5fe7b0dc66e1b8": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$34 ExpertModeSetting.php:34]", ./deployment/translations/projects/phabricator_ext/core/qqq.json: "e20bdc892a74ee50": "Used in:\n\n[https://phabricator.wikimedia.org/source/phab-extensions/browse/wmf%252Fstable//src/expert/ExpertModeSetting.php$12 ExpertModeSetting.php:12]", grep: ./deployment/.git/modules/libext/misc/index: binary file matches ./deployment/libext/misc/src/expert/ExpertModeSetting.php:final class PhabricatorExpertModeSetting ./deployment/libext/misc/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'src/expert/ExpertModeSetting.php', ./deployment/libext/misc/__phutil_library_map__.php: 'PhabricatorExpertModeSetting' => 'PhabricatorSelectSetting',

