EDIT: This question does not relate to human-shaped bipedal robots. Those are obviously impractical due to the square-cube law. Vehicles with multiple limbs have been seriously advanced for climbing and space exploration. What other circumstances would justify this design?
- Ability to traverse rough terrain. Tracked or wheeled vehicles aren't capable of crossing some divides. Larger legged vehicles could climb over walls as well.
- Squid type pump propulsion could be very stealthy.
- Multi-legged ant- or spider-like vehicles could potentially be used as 'forklifts' in rough terrain, for construction, mining, or forestry. Their ability to accelerate in any direction from a stop, and climb over obstacles would give them some advantages over tracked vehicles. Taken to the next step, this could be useful for space exploration.
- Multi-legged vehicles that can climb could be useful for vertical construction, although its hard to see exactly how they'd be more useful than a crane. Taken to the next step, this could be useful for digging vertical bores into ice planets like Europa.
- Flying-insect based aircraft won't work in Earth's atmosphere when scaled up. Flapping wings don't provide enough lift.
- Squid are optimized for traveling underwater, not on the surface. It takes more power to travel on the surface, and the squid's water jet propulsion probably can't provide enough power. I assume that there is no good reason for merchant vessels to be submarines, so most ships will still travel on the surface.
- Wheeled vehicles will move faster than anything with legs.
- As long as we humans are building roads, wheeled vehicles will be able to use their speed.
- As always, complexity is a bad thing. The complex joints of legs will always be harder to build or maintain than a wheeled or tracked vehicle.
These machines are cool, but also very expensive. The primary benefit is of course the same benefit that humans and other legged creatures have over tanks -- They can easily maneuver over rugged terrain that treads could not be able to deal with. That being said, in a world with aircraft (Although b.Lorenz points out not all celestial bodies have an atmosphere capable of supporting aircraft), there is no real reason to go with n-pedal tanks.
If you really are interested though in such a device, I suggest you look at Boston Dynamics new Hybrid robot, and compare it to some of their previous inventions like the Big Dog. By using a creature with knees, but using wheels instead of feet, the hybrid has the advantages of a bipedal locomotion, while eschewing the complexity of multi-jointed ankles.