By appointing a coworker as the default approver, you make sure that every document that does not activate any workflow is always sent for approval to one selected coworker. If you have turned the Workflow component off, for example, a default approver lets you make sure that every new document has to be approved by the coworker you select.
The default approver can be selected by going to Settings > Advanced > Workflow and then selecting the coworker you want as the default approver:
