I have always wondered why animals use terrestrial locomotion?  Why don't they just disappear and then reappear wherever they want to be?
It is definitely an interesting topic! I believe the way the body is supported by the legs is an important part. 
Yes, my appendages are certainly helpful in giving me an upright posture.  However slime molds also move, and they don't have legs.  They must be teleporting, right?
the posture, the way the body is supported by the legs, is an important aspect. There are three main ways in which vertebrates support themselves with their legs – sprawling, semi-erect, and fully erect. Some animals may use different postures in different circumstances, depending on the posture's mechanical advantages. There is no detectable difference in energetic cost between stances.
The "sprawling" posture is the most primitive, and is the original limb posture from which the others evolved. The upper limbs are typically held
of terrestrial locomotion, it is the basic form of locomotion of two major groups with many terrestrial members, the vertebrates and the arthropods. Important aspects of legged locomotion are posture (the way the body is supported by the legs), the number of legs, and the functional structure of the leg and foot. There are also many gaits, ways of moving the legs to locomote, such as walking, running, or jumping.
Legged locomotion Posture.
Appendages can be used for movement in a lot of ways: