It can be anything really. Spikes in electricty seem to be the main culprit.
My logic goes, the bios tends to be the slowest part of booting up, once the OS is up it doesn't need to load again. Once I open a program I'll want to minimise it regularly but not really open and close it, so the windows swap file on an SSD makes sense.
I use quite a few programs that need to cache or access large files quickly, so I would direct their cache settings towards the SSD, knowing that if the SSD did go pop, my scene file, assets, emails and lost time would be minimal risk.
There's not a lot of performance boost you could give to TG with an SSD, you might shave a few minutes off an animation accessing the GI cache, large .ters or textures. RAM is the main thing, larger textures, .ters etc.