One theory as to why humans began walking upright notes that when our species was just getting its start, reaching up for fruit was a good thing to be able to do.
Those who could temporarily rear up on two feet could get more food, and thus, had a survival advantage over those who couldn't. Those who could stand on two feet indefinitely beat out those who could only do it temporarily. In time you have the modern, free-standing human form.
You may have noticed that human beings are good at doing a lot of other things than just picking fruit. These days we can stroll around on two feet at will. Our hands are free to do all sorts of things, from building space stations, to building radio stations.
Our brains have grown much larger than they were when we lived in trees. We have become a radically different kind of creature.
If you can get a sense of how this has happened, you will have understood something wonderful about evolution. First comes the competition over fruit. The animal that stands the best survives and reproduces, passing on its characteristics. However, now you have a lot of standing animals.
A standing animal can see predators from farther away, which is another survival advantage. Standing also means your forelimbs can be used to manipulate objects and create tools. One thing leads to another, and another, and another. Across millions of years, a little thing like grabbing fruit can trigger an entire change of species.