well, while I definitely block ads as well (when I don't reverse engineer them), this directive does have a good reason. It means:
"Allows a sandboxed document to open new windows without forcing the sandboxing flags upon them".
If it was absent, when user clicks the ad and it opens a new tab of the advertiser website, it would inherit the sandbox directives from the safeframe, which might break it. To be clear "sandbox" in this context refers to the iframe sandbox[0], not to be confused with the renderer process sandbox[1].