[dingleberry420]

Same reason you would deploy to plain old servers. Your stuff has got to run somewhere. And (Google hosted) kubernetes solves a lot of issues for you out of the box, which you would otherwise have to solve eventually anyway. And it takes basically no additional maintenance, if you use a mature managed service.

You can have your developers manually copy paste files to a server, which is great while it works, but it's far better to have an automated process. At that point it's not really much more work, in fact I'd argue it's less, to pick docker + kubernetes than any of the alternatives.