Einstein wasn’t just a patent clerk, he was an original thinker in what may have been the perfect job. Einstein evaluated patents during an explosion of technological innovation. Visualizing how the devices and processes worked, whether they worked, and if they were original was the best possible practice/training for visualizing his own thought experiments, running those experiments and evaluating them. Had he possessed the extraordinary bad luck to have nabbed a great university position would probably have spent too much time mulling the unoriginal thoughts of his “superiors” at the university, bending to their ideological requirements or fighting with them, and would never have had the opportunity to race through his assignments at the patent office and spend the rest of the day “skylarking” about weird questions of physics.