Why did the label on my Fig Newtons get redesigned? It's not because it lacked in usability. It's because the company knows that customers respond to visual updates. If something looks the same year after year, lots of people will see it as old and tired. A redesign makes it feel modern and fresh. At least to enough people that it matters.
I had the same experience on a recent trip to The UPS Store. The color scheme was all different from the previous time I shipped a package. I commented to the clerk that they'd redecorated. He said they do it pretty often.
Software has a visual aspect to it, and the same forces are in effect. Part of a UI designer's job is to be on that treadmill. That's probably not what the job should be, but the people who pay their salary want it to be.