When you slide a magnet off a plate, you are doing the same amount of work as when you pull it directly off (slightly more, in fact, because of the friction). However, while it's the same amount of work, it's spread out over a much larger distance, and so requires much less force. When you slide a magnet off a plate, the force decreases gradually, starting as soon as the magnet reaches the edge of the plate. When you pull a magnet off a surface perpendicularly, the force is quite large when the magnet is touching it, but decreases quickly as soon as it is a short distance away.
The basic principle is the same as the one that makes levers, double pulley systems, and switchbacks useful. Since work = force × distance, increasing the distance reduces the force required.