It's much like how the design of Facebook constantly changes without adding any sort of benefits. I think that they have large design teams who have nothing to do because the product is already perfectly adequate, so they invent work for themselves.
I think it keeps more than the designers in a job. Half of these changes come with some exaggerated prose from a marketing or design consultant, explaining how their new logo or colour scheme or layout really speaks to the fundamental nature of the universe and our place within it, and how suitable that is for a chat client or an analytics platform. Throw a custom font in there too for good measure.
