On tumblr at least [dunno if it actually originated from there but it's used a lot], they're not so much white bars as they are usually three different sets of gifs of the same scene that are synced together to get by the individual size limit.
Some basically edit wider gifs to get that pseudo 3d effect.
I don't get it. Is there something special about those 2 vertical white bars?
Those bars cover the background, but foreground goes above them. In motion, depending on a particular animation, it may create an illusion of 3D. Like on the gif I linked the arrow is first seen behind the bars but then it goes above, thus making it look like it gets closer to the viewer. But it doesn't work well on a static image as we can see here.