I think it very much depends on what you're searching for. If you were searching for a mortgage the other day, that makes you a more valuable user (based on the assumption that a mortgage ad will pay a lot more than other ads) than if you were looking for a lower value product.
If you'd visited a transactional e-commerce website and didn't buy anything. As an advertiser I could re-market to you with a discount code to try and entice you back. I know that you've expressed an interest in blue widgets and I know that you didn't checkout on the site.
From a publisher point of view, it just puts more ads into the auction for that impression. If you've already got a site in a high paying vertical it probably won't outperform contextually relevant ads. I think the real winners are site owners who had previously had much lower eCPM who now have far more potential ads to show to users.