As you use and configure and modify the system, it starts to drift into a functionally random state. An example: directories of plugins/etc. where the update w

As you use and configure and modify the system, it starts to drift into a functionally random state. An example: directories of plugins/etc. where the update won't have any content, but content exists in the current install. Upgrading/overwriting the app wouldn't 'catch' these directories of stuff, without having/keeping track of every place in the entire fs that functions like that. A checksum would let you know that the directory is different, but not 'how', per se. By just nuking the whole fs from orbit and unrolling a new one, you have a 'known' state without any of that tracking internally (which is probably impossible to get right organizationally).