There’s no rule that says you have to wear padded liner shorts when you ride, but they definitely exist for a reason. Given that mountain biking takes place on rough trails and uneven terrain and we spend lots of time seated and pedaling on trail rides, adding a bit of cushioning between your body and your bike saddle can help to enhance and maintain your comfort while riding.
By adding some cushioning, padded shorts help absorb some impact, help to avoid pressure points, and can reduce or prevent irritation, chafing, and sores that may otherwise develop. Another bonus is that padded liner shorts are generally pretty snug fitting, and they can help to keep sensitive parts of the male anatomy tucked up and out of the way, reducing unwanted and uncomfortable encounters with the saddle.