yeah... try measuring out a tiny fraction of an egg... not easy. When I was experimenting with small batches I used a syringe to measure out small amounts of beaten egg!
The real problem with testing a single cookie is creaming a tiny amount of sugar and butter. Even if your ratios are accurate the preparation will be somewhat different and texture will be affected. But small batch testing is the way to go to dial in ingredients for ideal cost and flavour.
Just think about how annoying measuring a cup of peanut butter is, scraping it out of the measuring cup, washing the cup.. vs just adding spoonfuls of peanut butter to a mixing bowl.
You can still bake just fine with measuring spoons and cups but you’ll be more consistent and better with the scale.
They are required for experiments. I like to add a small amount of baking soda into my banana bread for a nicer crust but if you add too much, think 0.X grams too much, then the banana bread could begin to have a vaguely pretzel-like taste.
Just buy one.
Also - I would so like this site for brownies :-)