Packing cubes help in two distinct ways: organization and compression. Regular (non-compression) cubes recover modest amounts of suitcase space by eliminating air gaps between loose items, but the bigger benefit most travelers report is speed and peace of mind. Finding a specific shirt takes seconds instead of digging through a bag. Dirty clothes stay separate from clean.
Compression cubes go further: they squeeze bulkier items (sweaters, puffer jackets) and free up real space, which matters most for carry-on-only travel.
The cubes do not compress clothes by themselves. Packing technique matters more than the cube. Rolled or tightly folded clothes get the full benefit.