Opening in a new window is a red flag for ad blockers so the ad (in shape of a banner or an image) is more likely to be invisible to the user who has an ad blocker.
I personally hate it when I'm forcefully sent to a new window myself. That's bad UX, people who want that behavior do it themselves with the middle click or right click + open in new window.
It gets even more annoying on mobile.
Of course, since it's such bad UX, Yahoo use it in their SERPs.
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