IMO, Designer serves well for vector based illustrators, and Photo, for raster based (more of my case, tho I also do vectors) illustrators and digital painters (the distinction is not easy to make, IMO, among certain types of professionals).
Affinity Photo is good now for digital painting. (I wouldn't do digital painting with A. Designer, although is technically possible. IMO, Designer serves well for vector based illustrators, and Photo, for raster based (more of my case, tho I also do vectors) illustrators and digital painters (the distinction is not easy to make, IMO, among certain types of professionals).