The reasoning I have seen is that two logically adjacent sectors S1+S2 could be physically in the order S2+S1, so to read both sectors, you may wait one revolution for the data to be under the read head. Multiply by a zillion for a large file, and that slows things up. With an SSD, physical location doesn't matter -- you don't have to wait for something to move under the read head. So there is no advantage to defragging. The disadvantage is that each location on an SSD can be rewritten a limited number of times, which is not a factor with spinning drives. Although that limit might be rather large, in the petabyte range, but some optimizing programs spend every waking / inactive second shuffling data around so they could chew through a mega-write in a week.
Actually "spinners" also have limited number of bytes that can be written to any spot and can weaken it's magnetic properties. SSDs also use What is wear levelling of SSD drives? - Quora to spread number of writes to as many cells as possible. Some have an invisible partition for that while others just use whatever free space there is.
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