Nice explanation Yek, as usual.
For a person new to the modeling world or simply new to Fractal, if you are interested in using your Fractal WITH a traditional cabinet, then you would need a power amp, like the SD170. There are plenty of options actually...SD, Matrix, etc...
When connecting to a powered monitor, NOT needed.
Personally one reason I like the 170 is because it has EQ controls on the unit. So, quick tweaks are easily done at the SD unit without doing anything to the preset in the Fractal. This is especially nice when playing a variety of venues where slight bass and/or treble tweaks may be needed.
Also, it means you can send a direct signal to FOH (no tweaks) and still have your cab onstage connected to the SD170 and tweaked for YOU.
Many guys like having a cabinet on stage for the tone/feel. I did it for quite a while, worked very well.
Of course that's only until you go to full in-ear monitoring as a band and realize you don't have to lug that damn cabinet around anymore
Anyway, I've used the SD170 quite a bit in different situations with my AX8 and other Fractal gear and found it performs very well.
I've been able to run a 4X12 cab and compete with a drummer no problem. Of course YMMV (your mileage may vary).
Sounds like if you have a powered Alto speaker, you wouldn't need the SD170 though.
Still a nice piece of kit to have available.
Best of luck, enjoy!!!