Without an apply button, ArcGIS Pro is re-drawing the map for every symbology attribute change.
Example: Raster - Display Background Value - This part of the symbology pane has a check box and 3 fields to set the RGB values you want the change the display for.
Changing this sends 4 redraw requests to the map despite only 1 needing to be done.
I would prefer to have to click apply in all scenarios over having to deal with multiple redraws with no purpose.