Assumptions: Those who could see farther were more likely to acquire food, thus, standing up increased the human's range of hunting and gathering - to which we are the undisputed world champions of hunting/gathering. Also, by standing up, in increases the body size, making for a more dominating pose. This behavior also reveals the genetalia, so mating may have been a factor in our erectness. Also, being a naturally warlike species, we may have actively exterminated any other species which we considered a threat that also walked on two legs, such as Neanderthals. Our early ancestors may have hunted from the protection of a forest, requiring frequent vertical reaching. And there are many mammals that can stand or walk on two legs - bears, mice, gophers, raccoons, squirrels, dogs . . .

I will assume the question is why our walking on two legs is better than other animals who can walk on two legs. Creatures that walked on two legs with our unique structure were other human like species who went extinct. I would guess because they had less of a brain that could not adapt. Humans are not the only ones who walked on two legs on earth. Some apes developed the special a bone structure ( seen only in humans today) that enabled them to walk on legs. As previously pointed out, other animals can walk on two leg also. What makes a human different (and better) than the rest that can walk on two legs is the ability stand on two legs for a very long time. The human foot is also a unique design, a arch that aides walking on two legs for a very very long time (no arch is called "flat feet" and the army won,t take you). Only humans can stand there for a long time by locking there knees (no animal can lock their knees). Walking on two legs became a need as the disappearing forest led to wide areas of grassland. The ability to see over the grass was a lifesaver. Two legs has it own set of problems that come with the package. The infamous back problems and hemorrhoids are some exclusive to us. I excludes animals that walk on for legs only in this answer.

Because we have evolved from Aquatic apes. Actually using only two legs has significant disadvantages on dry land - running is much more efficient on four legs, the organs are less exposed when on four legs. However, get into the water and standing on two legs has advantages. Another good question would be why do human apes have no fur, why has our larynx dropped?

Nice question (+2)

Actually, most of the fossils have been found near water. Africa was a very different place to now. AAT does not suggest that we were like fish, rather that we spent a lot of time in water. What is your explanation for why we uniquely amongst apes, have no fur, walk on two legs, can speak, control our breathing, and are the only ones with subcutaneous fat?