The Pro branding is clearly a simple market segmentation.
Ever since the iPhone 8, Apple really wanted the regular customers to get the regular iPhone (in this case, the 11). Thus the branding of just iPhone 11. It is THE iPhone for 2019. The IPhone 11 Pro is the premium/cutting edge model for customers at the ultra premium segment. The XR and XS branding were a bit confusing because Apple already used the iPhone X in the previous year. This year, it’s more clear cut, and the bigger price gap between the 11 and 11 Pro shows the intention. Apple really want to segment their customers.
The same thing with the iPad and iPad Pro. The price difference between the regular iPad Air and the iPad Pro is huge.
Lucky for us, at least on the iPhone side, the iPhone 11 uses the same exact chip as the Pro. I can see Apple differentiating these segment even further in the future. I can see the regular iPhone getting milder refreshes each year, while the pro iPhone gets most of the bells and whistles (with the price tag as well)