Disc Brakes. It's About Control, Not Power.
Many cyclists assume the biggest reason to switch to disc brakes is to gain stopping power. While hydraulic disc brakes on a road bike would almost certainly be more powerful than existing rim brakes, the bigger benefit is actually that cyclists would get control over the available power.
Some tech talk
In its current form, a rim has many jobs. Not only is it the braking surface, but the rim also anchors the tire and helps support the rider. And, as a relatively large, rotating object, it can't be too heavy. To meet this requirement, rim manufacturers use materials that are strong and light, but don't offer ideal braking performance. By contrast, a disc rotor is small, so even if it's made from a relatively heavy material (most are stainless steel) it still winds up being fairly light—an average 160mm disc rotor weighs about 115 grams, while a standard aluminum box-section clincher rim weighs about 440. The greater degree of control comes as the result of a bit of counterintuitive physics. A disc rotor's smaller diameter compared with a rim's brake track means it has to work harder to stop a bike. But because it's working harder, you get better control (modulation), explains Wayne Lumpkin, founder of Avid Brakes and creator of the Ball Bearing mechanical and Juicy hydraulic disc brakes. How much harder does it have to work? According to Lumpkin, disc-brake pads must squeeze with about 1,000 pounds of force to achieve near-lockup, while a rim brake needs only 200 pounds for the same job. The larger span (0 to 1,000 pounds versus 0 to 200) is a bigger window in which to control braking force; hence, better modulation. Then there's the superior feel of a disc brake. The calipers found on most road bikes are relatively flexible and are mounted to a bicycle with a single small-diameter bolt. Their job is to squeeze rubbery pads against a compressible brake track. A disc, by contrast, uses a stiffer caliper and squeezes largely noncompressible pads against an incompressible rotor, giving you a solid, precise feel. Cyclists used to a road-bike caliper may find disc brakes grabby at first, until they adjust to the increased power and learn to take advantage of the precise control available.