I'm sure there were enumerable back and forth discussions about that. Bing differentiated itself in the beginning with the large background image on the homepage as opposed to the simple colourful logo on white background that defined the Google experience.

Then Google copied the background image idea.

Now Bing has gone minimal, but there's obviously still some residual pull/desire around using imagery.

That little strip is a remnant. Like a human appendix it's a residual organ that wasn't completely eradicated as it evolved into the current design.