My system disk is already a 240GB SSD - more than enough for OS and Apps.
However, like lots of people, I have a large raid array (currently 3TB and soon to be increased) that holds data files.
I tend to access the same files from the array, for days or even weeks at a time, before moving to a new project that I'll work on for an extended period. There are also some data files that I always use on a daily basis. I want to have those files open as fast as possible.
I can address this with a small cache SSD that updates with my usage pattern, while the system SSD is free to do its own thing..