When removing users from your Looker instance, it is highly recommended that you disable these user accounts, as opposed to deleting them. This can be done by navigating to Admin > Users, selecting Edit next to the user, and changing their account status to Disabled.
This will prevent the user from logging in, but will avoid the loss of this user content, schedules, and data. Disabled users will also not count toward the number of seats allotted to your license.
When you delete a user, the user's personal space is deleted, thereby moving all of their Looks and dashboards to the Trash folder. Admins can recover deleted content from the trash before it is emptied, and if the content was originally in the deleted user's personal space, it will go into the space of the admin that restored it.
Any schedules that a deleted user owned will also be deleted, so it is important to reassign schedules you want to keep, before you delete the user.
Lastly, historical usage information for the user (which can be explored via the Admin > Usage page, as well as through i__looker) will be lost if the user is deleted, as opposed to disabled.
For these reasons, it is highly recommended that you disable, rather than delete Looker users!