Memory and other upgrades for photo editing

as the others stated, they do.  And if the drive gets close to full, this can actually start to hurt performance, so you do best to keep the last 20% of capacity (plus the typical 7% spare space) free.

But why it's not a problem is because the typical latency to read a block is 0.1ms, whereas hard drives are between 4ms (the 10,000 rpm Raptor line) and 14ms for the green 5400rpm drives, with the classic 7200s in between around the 9ms average.   So even if it is fragmented badly, the SSD can do 40-140 seeks in the time it takes the hard drive to do one average seek.

And that's most of the reason for the better overall performance.  SSDs can do thousands to tens of thousands of i/o operations per second, the spindle drives struggle to get to triple digits.  If you were just streaming a large file, like copying a dvd iso, the performance delta drops substantially.