failboat :
It won't decrease longevity like a SSD will but random reads get slower when there is more space filled for it to check. You should be defragging spinning disks periodically. That will move related blocks closer.
Okay I believe I understand a little bit better now.
So getting kind've off the original topic somewhat but just to cement the concept...
When I defrag a disk, you're saying I'm moving related blocks closer. As in (correct me if I'm wrong), I'm moving say...memory blocks of a program that may have been scattered through regular use of the drive, and condensing them so that when the drive goes to read said program, it doesn't have to jump all over the place to find the required data.
Also, when I'm defragging, am is it simply condensing those memory blocks or is it also physically moving those condensed blocks closer to the center of the drive, so that the data can be accessed faster? (Or does this also depend on how the drive is constructed, or am I wrong all together?)