I've used Studio Instruments to play tracks my own MIDI songs. For example, I open the browser view, click insert synths, choose Studio Instruments, then choose SI-Electric Piano, select Simple Instrument Track, and the SI Electric Piano gets assigned to my 1st MIDI channel, which is usually keyboard accompaniment of some sort. Any of the 4 Studio Instruments can be assigned to your MIDI tracks.
You can also click on the name of a pattern inside the pattern browser on the left side of any SI window, drag it into your sequencer and drop it into place.
So, I guess there's 2 ways of using SI - assigning them to existing MIDI tracks or dropping patterns into a sequencer.