API page: https://api.drupal.org/api/drupal/core!lib!Drupal!Core!Entity!entity.api...
A common use case for hook_entity_update(), hook_entity_presave(), and the associated hook_ENTITY_TYPE_*() hooks is acting if a certain thing has changed.
For that, you need to be able to get hold of the original copy of the entity. It's available in $entity->original. This should be documented.