Pro needs the ability to display dynamic text for symbology labels, which will then appear in the legend. That is, it should be able to symbolize map features based on one attribute, then display a different attribute in the legend. The reason I can't just symbolize on the attribute I want labeled is because it's subject to change, but I don't want the symbology to change.
Here's an example use:
I have a layer showing zones for enforcement officers. The zones are symbolized on a field with values 1, 2, 3, 4. But I want the officers' names displayed in the legend, which are stored in another field. The names of the officers occasionally change, so I can't symbolize on them without having to fix the map. Zones 1, 2, 3, 4 never change and can be reliably symbolized upon, but I have to manually update the names in the labels. I wouldn't have to if I could put dynamic text in the symbology label specifying the name field! Like this:
