If you are wondering why your cookies come out different every time you bake, it isn’t due to variance of temperature and humidity – IT IS BECAUSE YOU ARE USING WILDLY DIFFERENT AMOUNTS OF FLOUR.
And yes you ducking can tell the difference between a batch of cookies where the flour is weighed vs scooped.
You can’t accurately measure flour by volume. The amount you get in a scoop will vary depending on how compressed it is. You weigh flour to remove that variance, which can be far greater than 5%. Don’t believe me? Put a cup of flour in a measuring cup, then start pressing on it to pack it (you won’t have anywhere near a cup anymore). Controlling for flour density (ie: consistently measure by volume) is nearly impossible.
Brown sugar is similar but easier to manage (most recipes tell you to use packed measures instead of scooping).
Things like white sugar, sure – scoop away.
We lost power for 3 days back in 2006. The water heater still had hot water in it when power was restored. It wasn’t piping hot that last day, but was still good enough for a shower. I was quite impressed.