Why add baking powder when creaming sugar and butter?
I can't think of a reason to add the baking powder before creaming, but I can think of a few reasons not to:

Pretty much all baking powder these days is "double-acting". It provides leavening two different ways, under two different conditions: first, when it comes in contact with moisture, it produces carbon dioxide, and second, when it comes in contact with heat (in the oven). Adding the baking powder earlier than usual in a cookie recipe would trigger that first leavening earlier than normal, as soon as the baking powder comes in contact with the water in the butter. It's normally added at the end of the process so that as much of the CO2 as possible from the first leavening is still in the dough when it goes in the oven.

Perhaps more importantly, adding the baking powder to a creamed cookie dough very close to the end of mixing, along with the flour, keeps the agitation of the beater from beating too many of the carbon dioxide bubbles out of the dough. That's normally the first time the BP encounters moisture and starts that first leavening, and ideally not much mixing takes place after that step. Adding it before the creaming step begins would mean that any CO2 bubbles introduced by the first action of the baking powder aren't going to stand much chance of survival through the rest of the process, as the butter and sugar are creamed, eggs beaten in, etc.

Also, baking powder doesn't last forever, its leavening power decreases with age. Yours may just be old, or at least old enough that those two other factors were enough to affect the end product.