Post-style pads for centerpull and cantilever brakes allow easy adjustment of the pads for wear: Loosen the nut that holds the pad, slide the pad further inward, align it correctly with the rim, and tighten again.
This is better than shortening the brake cable via a barrel adjustor, which changes the height at which the pad hits the rim. On cantilever brakes, the pad will hit the rim lower as it wears, until it dives under the rim, and you lose all braking power. On centerpulls, the pad hits the rim higher and higher, until it cuts into the tire, causing a blowout. Sliding the pad inward maintains the geometry of the brake and avoids these problems.