In fact the benefit is not for those outside the tower but those inside it. Ordinarily, we see (and ignore) reflections in glass all the time, for example from computer monitors or car windows. But air traffic controllers must not have any distracting reflections as they monitor flights. By tilting the glass away, any errant light from inside the tower (such as video screens, lights, etc.) are reflected up onto the ceiling, which is painted black.
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