Human function is actually very effective. Look at the kinds of motions we are capable of... such as in Wushu, Gymnastics, Martial Arts, Pole Vaulting, Parkour, can hop on and use other transport means (like a bike, motorcycle, skateboard, skis, boat, car..etc) , climb mountains, walk on a thin tight-rope or similar thin width path, etc...
Treds can get off track. Bound Up. Shreded. Total system is heavy. Takes up a lot of space. Uses more energy. Cant navigate all forms of terrain and high grade slopes (or vertical faces). Not usually very swift. Lots of friction.
Wheels of course, work great on smooth surfaces, but even they fail to do what the human body can accomplish.
A 4 legged animal can have incredible speed, but that can put limits on carrying capacity, as well as weapon height. Its also not as efficient as two powered legs over four.
The thing about walking that makes it so efficient, as that it only requires partial energy. Part of the job is left to gravity. A constant state of falling, with minimal balancing efforts.
These kind of bots can be far more agile, and harder to destroy than something like a more static rolling tank bot.
They can also work as a team, to perform coordinated feats that one does not even want to contemplate. Think cheerleader / acrobat style pyramid adders, and throwing formations.
Radio controlled drones are already killing people in the middle east...
A robot army, is a very scary, and very real possibility.