What is the purpose of the "default invoice payment page" field in the CiviContribute Component Settings page?
"Default invoice payment page" setting is used to set contribution page that can be used to pay pending or partially paid contribution using payment processor. 

More about the setting and uses: 

Wiki: https://wiki.civicrm.org/confluence/pages/viewpage.action?pageId=346750980

CRM Issue: https://issues.civicrm.org/jira/browse/CRM-19263

HTH
Pradeep