From what I understand, fragmentation mostly happens because some file gets deleted, and then when a new bigger file is created, it doesn't fit in whatever space is available, and therefore gets split up in 2 fragments
If files that haven't been modified in a long time, the chance that they will get removed or changed any time soon is very low, so if all files with an old modification time get moved to the start of the disk, then the chance of "holes" appearing there is lower
I could be mistaken, but I think that if all old files will be moved to the start of the partition, and newer files (often log files or temporary files) are grouped together further down the partition, there will be a lot less fragmentation