If there's a game that truly defines what this feature's trying to get across, it's Battlefield 2. Play this squad-heavy shooter by yourself and you just haven't lived. There's nothing like fighting for your life with 64 players set across a sprawling battle map loaded with tanks, jets and all manner of soldiers trying to bring them down.
Online gameplay is one thing, but to really channel the heart-popping excitement directly into your brain, you're going to need a LAN party. Cram enough people in one place, all planning strikes and counter strikes with the very best military technology around and you've got a curse word factory ready to explode.
Basically, yeah, you can completely play this alone. Solo's certainly available, and no one's got a gun to your head to go multi, but you'd be damn crazy to wage this war alone.