Well, as a bike geek - who wrenches on them, changes old bikes to different configurations etc - I can visualize every part because I've dealt with all of them. I can tell you, for example, that ancient Shimano downtube shifters are held in place by an M4.5 bolt. M4.5? Try to find something to fit that at your local hardware store (when changing said bike to handlebar-mounted shifters). Or which way the opposite sides of a BSA bottom bracket are threaded (from memory! Which side has the backwards threads?) Or the whole stack of bits and pieces that make up a headset (both threaded and threadless).

Whereas a pro rider can probably tell you all about the biomechanics of how to optimally interact with the bike, the right foods to eat and how much to sleep and when. But the actual wrenching around with them? That's the pro mechanic's job.