Geist's primary purpose is as a SS, so it was not a complementary component for me, but a replacement. Deep down Geist is more what I would consider an EDM DAW, with 8 audio engines, FX banks, sample/slicing/layering capabilities, and the ability to sample audio internally (via its "Spitter" plugin).
On the surface, step sequencer is its most obvious use, but it is far more than that... All engines/patterns can be fired off real-time with MIDI if you want to delve to that level of detail.