Originally Posted by SaintDanBert
File systems and various configuration files use linked lists or some other structured data. After many cycles of add-change-delete, these data structures often require "garbage collection" and other reorganization. This processing is difficult to program correctly for all of the possible conditions and ways that things can go wrong.
As a result, it is frequently easier to read data from one file and write a new fresh well organized file.
Since new software editions often modifiy internal file details and implements new behaviors, a fresh install writes this new file with its new contents ... repeat for all of the various files of the new edition. Similarly, reading any file and writing a fresh copy often results in fewer file allocation extents. These multiple extents are called "fragments." We have all heard of defragmentation utilities.
Hardware evolves at a rapid pace encouraging acquisition of new and larger disk drives. Distributions also evolve at a different and similarly rapid pace. Given a workstation that is used routinely for real work, it is often convenient to apply an updated distro to that newly purchased larger faster drive.
All of the above is much different and differently motivated than the "re-format and re-install" that is routine with win-dose. Someone else might have the unvarnished truth, but I believe that win-dose often gets confused because there is no effective way to have multiple editions of DLL (dynamic link library) and similar resources. Given the need to apply updates and patches to win-dose every couple of weeks, after a while, a fresh install clears the confusion.
I had an "ext2" drive in service for 10 years with only 0.8% fragmentation at the time I took it out. No re-installs had been made.