> You still don’t know whether the customer would have ultimately found you without the ad.

Sure, maybe 100 customers would eventually find you. But if you buy the traffic, you can make them all find you on the same day.

> But other than gross spend changes there’s no way of telling for sure that the ads do anything.

I'll use the most fundamental example:

If someone clicks your ad and buys the product during that session, you know the ad worked. If you keep increasing your ad budget every day, and you are consistently returning $2 for every $1 you spend, you know it's working. Turn off the ads, and the revenue goes away. You'd be surprised how many people make their living doing this.