It doesn't matter what you wash your face with as long as you get it clean, right? This no-nonsense approach might work for washing a car, but your skin is a little more complicated. The bar of soap you use to get the motor oil off your hands isn't necessarily the best choice for cleaning your face.
One problem with washing your face with soap is that soap's pH is usually alkaline, while your skin's pH is slightly acidic. The pH of a substance is a number from 0 to 14 that tells how acidic or alkaline the substance is; water is neutral, so its pH is about 7. This pH difference sounds like a pretty minor issue, but it can cause two big problems. First, a mildly alkaline environment makes a great breeding ground for the bacteria that cause acne -- definitely not an ideal situation for your face, especially since men tend to be a little more prone to longer-lasting acne than women [source: Wu].
Another problem with soap's alkalinity is that it interferes with one of your skin's primary functions: to serve as a barrier. Your skin's chemistry helps it act as a wall to keep bacteria and other invaders out, while keeping moisture in. Changing your skin's pH can damage this barrier and let too much moisture escape, leaving your skin dry and irritated. Add this to the fact that the skin on your face is more delicate than the rest of your body and that most men shave daily and you're asking for trouble [source: Wu].
Besides the pH problem, many bar soaps have added ingredients that are irritating to your face. Deodorant soaps, for example, often have strong fragrances that can aggravate your skin [source: American Academy of Dermatology]. Many men who have oily skin or acne make the mistake of thinking a "stronger" soap will solve their problems, but irritating your skin makes acne worse and causes redness and itching, to boot. You're much better off sticking with a gentle product that addresses any skin issues you have, without creating more problems. Soap also leaves "soap scum" on your skin, which doesn't feel very comfortable [source: Wu].