Clickthrough rates are one thing, but plenty of ads don't rely on the ad being in the actual chain of purchase. Ads for small stuff like movies, beverages, snacks, etc., or big stuff like cars, furniture, etc., try to get consumers to buy those things outside of the medium that the ad is being presented.

Plus native advertising when you're looking for a specific purchase can sometimes factor in. Someone might pay more for a particular hotel room to get more prominent placement in results, and I'm not going to intentionally ignore that sponsored placement when choosing between a bunch of hotels. Maybe the ad didn't actually make a difference (in theory my purchase decision would've considered that hotel anyway, and if it's the best for my needs then they would've gotten my business without the ad), but I've definitely purchased sponsored results when searching for a product that I already intend to buy.

And if it counts as an ad, paid referral links from recommendation websites I trust are an easy way to "support" an outlet that I use.