One has to wonder why Cursitor Doom thinks that he knows. He isn't either. Repair technicians typically have to work out why a device isn't working, which is a process of forming hypotheses and testing them.
Designers tend to come up with hypotheses faster than repair technicians, so they don't test them as thoroughly, but they do tend to fix things faster.
At Cambridge Instruments the design engineers frequently got called in when some expensive production machine wasn't meeting its performance tests, and we frequently did well. We cost twice as much per hour as the technicians, but a million dollar machine sitting on the production line didn't make any money at all until we could ship it out.
Our chief engineer got shipped to America once to a machine that wasn't passing it's acceptance tests, and solve the problem instantly by recognising the ancient hydraulic lift that took him up to machine under test.
It had an associated magnetic field due to the big lump of iron involved and our machine was sensitive enough to the local magnetic field that the lift going up and down messed it up.