Purchase Requests and Expense Reports with status Pending Approval can be approved by a user with an Administrator role.
In situations where the direct Supervisor or approver is unavailable, this is the alternative.
Please note there is currently an Enhancement request logged that aims to give users the ability to designate a secondary or alternate approver when the primary approver is unavailable.
Details of the Enhancement are as follows:
Enhancement 94460: Employee Center > Employee Profile > Allow approver on leave to change his status to "Unavailable" for a date range and designate an employee as the approver in his absence.