Placeholders vs. Resource Fields in an Output Modifier

I was wracking my brain trying to get this to work (unsuccessfully):

[[+pr_tab_1_title:is=``:then=``:else=`[[+pr_tab_1_title]]`]]

Then I changed the placeholders to resource fields and it worked:

[[*pr_tab_1_title:is=``:then=``:else=`[[*pr_tab_1_title]]`]]

All the documentation I read and forum entries use placeholders for output modifiers. But my placeholders were void of data. Can anyone explain? Why use placeholders and not resource fields? How do I populate placeholders with data?

Placeholders are used when you have some snippet that's aggregating a bunch of resources (getResources, pdoResources, Wayfinder, etc.). The placeholders go in the Tpl chunk for the snippet and are replaced by the snippet.

When you want the value for the current resource, you just use the asterisk (*).