It would be more difficult to use tools if we had to use all members for locomotion. The ones who started walking straight got an advantage for being able to fight with spears and bows.
But the spears and bows did not come along until well after the walking upright. We know that from the fossil and archeological record.
Of course, it's still possible that those who could walk upright could throw stones better I suppose? So the fundamental idea of tool use has merit. But the tools would have been far more primitive than spears and bows. And still, the walking would likely have happened before the stone throwing, so there's still a little uncertainty even with the idea that it helped us with primitive tool use.
I think it's probably more that you could see predators better and get a head start for the tree. But I have no data to back that up, it's just my gut hunch.
That's not totally true. We really have no record of anything except carved stone tools and pottery, because wood degrades. We have no real idea when humans started using wooden tools like spears.
Sure, there must have been a bootstraping effect, where a little better tools promoted 2 leg mobility a little, which promoted yet better tools and so on.
