In most complex jobs (that is, jobs with a theoretical component someone writes books about), the theory part mostly comes to the fore when something goes wrong

In most complex jobs (that is, jobs with a theoretical component someone writes books about), the theory part mostly comes to the fore when something goes wrong, and you have to diagnose and fix the problem. So, in this case, programmers would use this stuff when a sort takes up too much time or too much RAM for what it's meant to do. Then you need to know what big-O is and how to fix it. It seems a bit odd to spend all this time preparing for things that don't happen every day, but, really, that's what people pay the big bucks for: Someone who can smooth over life's little difficulties, at least in that specific realm.