I often use stack just to get a thick double tracked type of sound - or to widen the stereo field and get a binarual type of feeling... like the Poly Evolver or upcoming UDO Super 6.
First, finish up your single patch on Layer A, then copy it to Layer B. Stack them. Set Pan Mode to Fixed. On Layer A, set a Mod Slot to DC>Pan,-32. On Layer B set a Mod Slot to DC>Pan,+32. (32 effectively hard pans the layers L/R)
If you want to make some adjustments to parameters on one layer it will widen the stereo field - small adjustments to osc pitch, cutoff, envelope attack can separate the layers further, creating a huge stereo effect.
It works great for big pads, strings, and brass sounds, and also can be great for leads that will cut through the mix.
Another use case for stacks is to just treat the second layer like an extension of the first, to get access to two more oscillators + a sub. You can keep all parameters identical in the envelopes, filter, amp, effects sections, but use a stack to get access to 4 total oscillators + 2 subs... for massive osc stack sounds (stack four detuned saws with some lite chorus for a 90s supersaw type of sound).