A scheduled task enables plugins to execute some actions at specified times, without having to manually execute code at that time. WordPress itself uses some scheduled tasks to perform some regular actions.
However, some scheduled tasks may not be removed even if the responsible plugins are deleted from your WordPress installation. As you know, not all plugins care about the housekeeping of your WordPress. Hence, deleting these unnecessary tasks may help in cleaning your site.
It should be noted that cleaning scheduled tasks is safe as long as you know what tasks to clean. If you are not sure, it is better to not clean any task.
Use our plugin Advanced Database Cleaner to clean WordPress scheduled tasks. In the premium edition, you can even detect orphan scheduled tasks.