...because they would have access to data that is not theirs. I know we get so used to leaving our trail everywhere so folks don't care, but if you think about it, the Nest thermostat probably knows every time you've walked by it for the past few years, when you've interacted with the thermostat, settings / preferences, and a host of other data.

Left to the new owner, maybe "who cares!" but one should really try to get rid of data they no longer need / use wherever possible.