It keeps the vacuum a vacuum.
No evacuated space stays perfectly empty. Molecules migrate out of the materials in the annulus and permeate slowly through the walls over years. Left alone, that gas would gradually restore a conduction path and the vessel’s performance would decay.
The getter is a chemically active component that absorbs those molecules as they appear. Its capacity is finite, which means it is not a permanent fix but a consumable measured in years, and its size is a direct determinant of how long the vessel holds its performance.
This is one of the largest legitimate sources of price difference between two vessels that look identical on paper. A larger getter and cleaner internal materials cost money to build and they come back later as a lower evaporation rate maintained for longer.
