Climb categories are essentially a method of working how many points are awarded for the KOM jersey. For example, in the Tour de France, the first man over a cat 4 climb gets 1 points, a cat 3 climb award 2 for the first, 1 for the second, right up to HC giving 20, 16, 12, 8, 4, 2. HC and 1C are doubled if they are the stage finish in the Tour, as well.
Most one-day races don't give a mountains jersey (LBL is the only one I cnan think of), and when they do it is generally a minor prize to give the smaller local teams (like Landbouwkrediet, Topsport Vlaanderen, etc.) something to fight for when they have almost no hope of winning. As such, the climbs aren't usually categorised, and when they are, because they're irrelevant to most of the big teams, it's not mentioned that often.