I think the main differentiator is that Stomp mode is changing the Bypass state of a pedal, whereas Snapshot mode actually recalls a State of a pedal (this is in simplest terms). For example, say you had one Stomp switch (in stomp mode) that turns on 2 effects and turns off another. Then, the next stomp switch, you wanted to turn on a different effect, and only turn off 1 of the effects from the previous state. You can't really do that, since you are only affecting bypass state. In snapshot mode, you can completely recall a snapshot of effect/paramaters states.