It's not purely a marketing gimmick. Haswell and beyond CPUs have C6/C7 sleep states which means when you put the computer to sleep with these sleep states enabled, they use very little power compared to Ivy Bridge and prior CPUs. The problem is, this results in a very minimalist load on the 12V rail of the power supply. This is a crossload scenario, and power supplies that utilize a cheaper secondary topology, also known as group regulated, do not crossload as well meaning the voltages can actually go far out of the ATX specification. So usually power supplies that aren't group regulated and crossload better are C6/C7 compatible. They are usually indy regulated or use the modern DC-DC topology.
But be cautious. Sometimes they market a unit as "Haswell compatible" as a little trick even though it is not compatible with the C6/C7 sleep states. That is why it is always best to buy a power supply that has been professionally reviewed, so you know for sure. If you do buy a group regulated power supply, you can simply disable those sleep states in the BIOS and the computer will sleep happily.