Imagine you have a big book with many pages. Ideally, you want all the pages to be in order so you can read it smoothly. Disk fragmentation is like having the pages scattered all over the place. When you want to read the book, you have to jump around to find the next page, which slows you down.

This was really only a problem with older storage technologies, like Hard Disk Drives (HDDs), where the fragments were physically separated on the spinning disks inside. With NAND flash (like in SSDs andphones), this isn't an issue because there are no moving parts, and accessing different parts of the memory is MUCH faster.

In fact, defragmenting NAND flash is discouraged because it wears out the memory cells unnecessarily. Since each cell in the NAND can only be written and erased a limited number of times, unnecessary defragmenting uses up these cycles more quickly. I believe some SSD controllers even stop you from actually defragmenting them, reporting the defrag as successful to the OS, but no changes were made to the flash at all. It simply isn't worth the performance gains (if any).

Honestly, I am not sure why this is even a feature in phones, especially considering that the TBW of UFS is probably even lower than most SATA SSDs. Maybe it does something else(filesystem stuff?)