Better Splay & Less Springiness: The traditional bulb shape with shorter hairs around the edges allows for easier play and less springiness compared to the "S" series brushes.
Synthetic Alternative to Boar: If you're looking for a synthetic alternative to boar this is the closest you're going to get. The water retention, splay, backbone all lends itself to similar characteristics to boar. Since it's synthetic it'll never be quite the same of course, but for a synthetic, it's the closest I've tried to boar.
Base Diameter: 26mm (± 0.5 - 1mm)