1. Conceal Carry - It's hard to call out one benefit as the primary benefit, but the conceal carry and minimal print of the Magholder is probably one of the most popular! The horizontal mag carrier almost disappears along your belt, and even with a shirt pressed up tight against the Magholder would be hard for an untrained eye to know what is on your belt.
2. Comfortable - The problem that many mag pouches present, namely the vertical ones, is they can poke into your gut or side. Even if you don't have love handles like many of us do, they can be uncomfortable when sitting in your car or at your desk, or just bending over to pick something up. The horizontal nature of Magholder helps many of our users almost forget they have it on their belt. 
3. Fastest Reload - The design of the Magholder puts your spare mag at an angle that makes a pistol reload extremely fast. With traditional vertical mag pouches you have to awkwardly change directions with your arm or perfrom some impressive twisting as you grab the spare mag to then reload into the pistol. Whether pulling to the rear, or to the front, both directs are extremely fast and efficient. 

There are many benefits of carrying horizontal, many that we haven't named here. Let us know in the comments below what you think another benefit is that we didn't call out. Also, check out this awesome and quick video that describes the same benefits that we called out above.