The fade-in, as I understand it, waits until the page has loaded to actually start downloading the image. This means that the image download will not be competing for connections and bandwidth with HTML/CSS/JS files that are required for first paint.

Whether this makes a significant difference is probably highly dependent on the page and the user's connection, and I have no idea without profiling. But it's at least plausible that it would help some users on slow connections.

Of course, it could just be that devs are A/B testing time until the "load" event fires, in which case they've just found a way to cheat the metric.