Superfluous: it’s there for Android, but I believe that sans-serif will normally resolve to that anyway.
(I’m not certain about that, and can’t confirm it as I don’t have ready access to Chrome on Android but I got the impression some years ago that Chrome on Android uses the system font, which is Roboto. But even apart from that, the general idea is “stop specifying specific fonts and let the browser do its thing and the user get their chosen fonts, unless what the browser does by default is too bad, like Courier New for monospace”.)
Including its name doesn’t mean it’s available. Or that it should necessarily be used even if it is—when the purpose of including Roboto was to get a default font, you probably shouldn’t complain when you get a default font.
I suppose the reasoning behind having it might be that Android devices typically also have one of the fonts that appears lower in the stack, but before sans-serif... though that's not as likely if you're using a much smaller stack (also it's already quite near the end so... not sure why its typically used... could indeed be just trying to opt out of some manufacturers' undesirable alternative defaults, maybe).
