You can use the step sequencer to control anything that's controlled by MIDI and has an available MIDI input in Sonar, not just Session Drummer - you just route the track to the chosen synth's MIDI input and load step sequencer on that track.

Works with hardware MIDI gear as well, of course.

What it can't do is play audio directly - it outputs MIDI only, like the piano roll. You can convert step sequencer to MIDI clips and, with some limitations, vice versa. Basically it's another way of editing MIDI to the piano roll, that's set up to create the kind of regular, quantised patterns hardware (step) sequencers produce.