I think the intent is folks that aren't putting bins in the overhead bin (or in some cases check at the gate) load faster. You get on, you put your bag under your seat, you sit down.

Where I've seen it with US carriers and European carriers most often is when they have a full flight and they offer to let folks gate check their bags to their final destination in exchange for boarding early -- this typically extends to those without rollerboards as well. The goal being they don't miss their D0 target (or whatever target the airline has)