I have a challenging packing situation coming up and I considered not using packing cubes. Then I did a test pack and realized I NEED them. I'll have one standard and one compression.
My bag, like many, has webbing straps I can tighten - all good if I fold my clothes in a tidy pile. But I don't like folding anymore - when you pull one item out from the middle, it pulls on the adjacent items and makes wrinkles or otherwise unsettles the pile in some way - creating small dead spaces. So I do a modified fold & roll inside a packing cube so that every item is visible when I unzip the cube. I could do the same fold/roll thing without the cube, but most of my items wouldn't be captured by the compression strap. A standard packing cube is fine for corralling all my items.
I have a couple warmer items - very compressible fleece and knit items. I also have a challenging 8" deep requirement for my carryon bag. I will use my compressible cube for those compressible items, then position the center bulge under one of the suitcase compression straps.
Another reason to have compression cubes - souvenirs. Pack your outbound bag uncompressed. This keeps you from overpacking. Then, when you buy souvenirs, you can compress your cubes and add a few inches of space.
That's just for flying though. Once I'm at my destination, I won't compress the 2nd cube until it's time to head home.