A carb cap is an indispensable part of your dabbing setup that completes your dab rig and banger nail. This essential tool gets its name because it constricts airflow, creates convection and traps heat which allows you to dab at a lower temperature.
Terpenes in your concentrates are destroyed with higher temperatures and they are accountable for the flavor so it is best to keep temps low.
Most designs of carb caps are made to provide directional airflow so you can control exactly where you want the stream of air to go. Capping your nail also traps the heat and allows you a longer time for your dab session before it needs to be heated up again.