They're helpful for production purposes, where people may not have the skills to interpret schematics. Also, with more complex devices they're great time-savers in that they can identify wire routings and parts without having to trace out a circuit. But for figuring out how things work? Good luck. It can be done, if you make some assumptions and have a lot of time, but it's a crap shoot. What I've always thought was unusual is that for guitars, schematics are almost non-existent. They're pretty simple circuits so if you already know how they work then you don't need them anyway. I mean, who's ever seen a schematic for a house? They're essentially non-existent as well, for the same reason.