This product is compatible with the E3D cartridge style heater block on the Volcano.
The sock provides the additional benefit of insulating your heat source. This allows for more cooling of your print for better bridges and overhangs without the concern of accidentally cooling your nozzle and causing temperature fluctuations (or the dreaded temp fall error). It also insulates your part from the heat generated from the heater block which will cause some improvement in quality.
It is expected that this sock may become worn or damaged over time, and they're cheap enough that the shipping is more than the cost of one piece, so we've bundled them into multi packs. Your future self will thank us.