The reason why Windows MUST defrag an SSD is the same reason why it also MUST defrag an HDD:
“Actually Scott and Vadim are both wrong. Storage Optimizer will defrag an SSD once a month if volume snapshots are enabled. This is by design and necessary due to slow volsnap copy on write performance on fragmented SSD volumes. It’s also somewhat of a misconception that fragmentation is not a problem on SSDs. If an SSD gets too fragmented you can hit maximum file fragmentation (when the metadata can’t represent any more file fragments) which will result in errors when you try to write/extend a file. Furthermore, more file fragments means more metadata to process while reading/writing a file, which can lead to slower performance.”
The same statement is true for HDDs. It has to do with the metadata keeping up with all the file fragments, rather than putting together the actual file fragments. I believe if you want to stop the defrag process altogether you have to turn off the Volume Shadow Copy service.