[nvy]

For run'n'gun games like Quake, nothing really but for games with more controls like ArmA or Tribes those extra buttons come in handy.

Shield, nade, lean left/right, use item, etc.

[mrob]

Weapon switching is a big part of competitive Quake. Easy-to-reach weapon keybinds give you an advantage.

[nvy]

Sure but aren't they all on the number keys?

[elabajaba]

Sure, but try hitting 8 without moving your hand from wasd