Associating project fields with custom fields has a lot of advantages, including powerful search functionality and enhanced viewing capabilities.
Here are some examples:
Reviewing your project fields in almost all JIRA screens
You can see your project field when it is associated with a custom field in almost all JIRA screens.
JIRA does not provide us, on the Issue Creation screen, with the project on which an issue is going to be created. For this reason, project fields could not be reviewed on the Issue Creation screen.
Searching in the Issue Navigator by a project field
It is possible to search in the Issue Navigator for your project field when it has an associated custom field.
Viewing project field values in the Issue Navigator
It is possible to view in the Issue Navigator - neatly laid out in columns and rows - all the values for your project fields when they are associated with custom fields.
Working with project fields in any gadget, not only in Profields ones
With custom fields, it is possible to configure Gadgets that work with project fields.
Viewing project fields on Issue screens
It is possible to view project field values on Issue screens.