What happens to builds that were running when Jenkins crashes, or is restarted not in "safe" mode (waiting for running builds to complete)? Whether or not you are using High Availability to start another Jenkins controller, builds of regular projects that were already running will be aborted. The Restart Aborted Builds plugin will at least let you find and reschedule them, but for builds of projects which normally take a long time, perhaps hours or even days, this is not enough.
To address the needs of people who have builds that are just too long to interrupt every time a Jenkins agent is reconnected (or Jenkins is restarted for a plugin update!), CloudBees Jenkins Enterprise includes a plugin offering a "long-running" project type.
Refer to Administering Pipeline - Long-running builds for details.