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...