There is a situation where one field of a Drupal View needs to pull its content from two fields (aka cck fields) of a node. In my case, they were default author field and a custom field called contributor. This example uses Drupal 7 and Views 3.
If optional the contributor field was not empty, it would display the contributor and skip the author field. If there was no contributor, then the author, which cannot be optional, would be displayed by default.