Convenience. You can get your supps now rather that a week later. If I need like one thing real bad I will go to GNC but for large orders I always order off the net, much cheaper.
Plus, unlike the online stores, you can go to GNC and get the products, when they're marked down for clearance. To top it all off, if you have a Gold Card, you get 20% off that marked-down price.
I remember getting a bunch of IsoPure RTDs, marked down to $0.99 each. With the Gold Card, the price dropped even more to $0.80. Of course, my favorite time cleaning up at GNC is when they were marking down the 7-lb jugs of CELL-TECH to $20.
Waiting until they get reduced for clearance is usually how I end up trying MuscleTech products. That's how I tried out: ThermoGain, ThermoShred, Gakic (powder, now discontinued), Nitro-Tech (still get the bars and RTDs, when they're on sale), NaNO Vapor, Hydroxycut Hardcore, just to name a few.