This might turn into a bit of a wall of text, but I'm bored at work anyway hehe, so:
In a traditional Hard Disk Drive (HDD) you have multiple layers of the storage on a spinning type platter with a small magnetic head that actually does the read/write stuff on the HDD. It's best used for reading sequentially, because the head starts reading and the platter spins and the head keeps reading along that sequential track. Now if the data is heavily fragmented i.e. the data is not stored in this ideal sequence, the platter has to do some extra spinning and takes a little more time for the entire read/write to be done, which is why defragmentation is a great idea to do every once in a while.
On a Solid State Drive (SSD) however, you just have a bunch of flash storage chips sitting around and they can all be accessed and read/write at the same time. As a result the SSD actually intentionally writes data all over the place, trying to utilize as many of the chips as possible to be even faster. So an SSD is essentially meant to be (atleast slightly) fragmented.
So why don't we all switch to SSDs? As great as they are, they're not only more expensive than HDDs but they also have a lesser lifespan. So any frequent use small documents of yours such as music, office files etc might be better stored on your regular HDD for now.
Edit: Oh also defragmenting causes a whole bunch of unnecessary read/writes and doesn't really improve performance while decreasing life so that's why it's mainly not recommended. I realized I didn't even answer the question.
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