It is nice and comfortable in ArcGIS Desktop to have a predefined built-in set of polygon outline styles when one would like to simply and quickly differenciate some polygons from others by their outline styles. Also, it is useful to have a similar built.-in style, when just minor adjustments should be made on it to meet your needs.
This is more useful, when one would like to use complex composite outline style using style layers.
Alas, this possibility is missing in ArcGIS Pro and I would appreciate to have it in the coming versions!