It seems this is a better answer than many—there is rarely a ‘smoking gun’ single reason that a bodily design is selected for. There is usually a constellation of various bodily features that mutually reinforce a ‘successful’ species design. Also there are multiple reasons why a feature is selected — it could be that walking long distances, seeing further on the savannah, and using our hands are all contributors.

Humans are in several ways a constellation of unusual features that work together. We are odd in being relatively hairless mammals. It’s unusual we eat such a broadly omnivorous diet. Our heavy focus on intelligence is weird.

What seems to be the précis for our design is adaptability and sociability. We’re somewhat like orangutans (who also have comparatively long childhoods) in that we range over large distances, eat a lot foods, and have the intelligence to know how to adapt to these varying environments and foods. However, we’re more social than orangutans and range over wider areas.

No other creature in the fossil record or recorded alive at any point in time can throw like a human and fashion tools to throw like a human with that curved and fragile radius and fragile shoulder held together by ligaments and tendons. It's not just the standing and the walking, the back pain, and the twisting foetus for birthing - the tool use outweighed the disadvantages. Cave guys standing up and bumping in to the stone shelf to grab the good cave drawing soot cursed but kept the verticality.

I also heard that standing on two legs allows us to support a heavier head (and thus a bigger brain) which enable us to become smarter than other animals.