Because that is the way it has always been done?Why do you want to push it up at all.
Someone taught me that, then I figured out it was a waste of time. Now I let the bar sag, adjust the chain then tighten it up, I've never had a problem.
Am I missing something.
I think the idea is that despite best efforts to keep it in place, the bar will move a little. Since most cuts are made with the bottom of the bar, that will tend to slightly push it up. But pre-pushing it, you account for the chain slack that movement would have otherwise caused before tightening everything up.