Well, I don't know about anybody else, but when I finished my OW cert, I felt like I had been given a license to kill myself

There was no way I felt competent to go off and dive with anybody, anywhere (or even in the places I had already been). I still had major buoyancy problems, difficulties with descent and ascent, and an almost complete inability to navigate anything but a single straight line.
Doing the additional dives for the AOW just really gave me more time in the water under instruction, with a structured dive plan. Although each dive had a specific purpose, they all worked on buoyancy and nav skills as well to some degree.
My instructor made an interesting comment right before our last dive together. He said that diving with students is like diving solo, and that's why he makes sure there are a lot of eyes in the water when he does it. It made so much sense. When you are newly OW certified and ask someone to dive with you, you are almost asking them to solo dive or worse, since they are not only responsible for themselves, but for you, too. At least instructors are paid to take that risk.