Up to 30% of the PPL exam is BAK (section 1 of the DAY VFR syllabus), Call it CASAs way of making sure you really did pass your BAK legit' and didnt have an instructor hovering over your shoulder
It's also completely possible to gain a license without doing the BAK test - if you elect not to bother with GFPT and go straight to PPL, you don't do the BAK, so the knowledge requires covering in the PPL?