The silicone covers for the E3D V6 match the block with the PT-100 sensor.
These also offer a huge boost in temperature stability, particularly if you use high airflow cooling around the nozzle to cool the print. The additional cooling combined with reduced radiation of heat from the block and nozzle to the print really boost the ability to print great overhangs and bridges, all while keeping the hotend temperature extremely stable.
Socks don't last forever. Depending on what materials you print and how much abuse they get they eventually wear away at the nozzle tip. Even with sticky abrasive materials and a bit of abuse they should easily last 100+ print hours. With less sticky materials like ABS and gentler use you can get almost infinite life.
