What does sealed mean in regard to the regulator?
DINO C Sealing the first stage from the environment keeps contaminates and other wear causing junk from affecting the operation of your first stage. But most importantly, it keeps freezing water from locking up the moving parts of your first stage on cold/freezing water dives.
The seal is a barrier between the piston style or diaphragm style scuba regulators that will keep parts from interacting with water. If you plan to Ice Dive, look into a sealed first stage.