When the World determines that a World Cell has no Users, it is marked for removal, which normally results in the Cell being sent to the Chunk Manager so that its Asset Chunks can be detached and possibly unloaded.
Before being sent to the Chunk Manager, however, the World checks to see if the World Cell's Asset Chunks can be recycled/reused by a new World Cell that is in need of Asset Chunks, the benefits of which should be obvious.
In order to make the decision on whether the Asset Chunks can be reused by a different World Cell, the Chunk Reuse Judge assigned to the Chunk Manager is used.
--Special Note--
The Chunk Reuse Judge is only used if Chunk Reuse Possible is enabled on your Chunk Manager! In addition, reuse will only occur between World Cell's that are using the same quality LOD Assets and which are on the same World Grouping.
Please note that under the hood, the World automatically creates some default judges when no judge is assigned to a Chunk Manager. There are two flavors of these default judges and which is chosen depends on an LOD Group's Use Single Chunk Set For All Cells settings.
1) When Use Single Chunk Set For All Cells is disabled, the reuse judge will only allow reuse when two World Cells have matching Streamable Grid Indexes.
2) When Use Single Chunk Set For All Cells is enabled, the reuse judge will allow reuse between all World Cells no matter their Streamable Grid Index, which is the same behavior as the Reuse All Chunks Reuse Judge.
If all LOD Groups using a Chunk Manager have the same value set for Use Single Chunk Set For All Cells and you are satisfied with the default reuse logic based on that setting, there is no need to create and assign a Chunk Reuse Judge to the Chunk Manager.
When a Chunk Reuse Judge is assigned to a Chunk Manager, its reuse logic is used for every LOD Group utilizing the Chunk Manager. If this behavior is not desired, you will need to create other Chunk Managers that do not use the same Chunk Reuse Judge.
Reuse is a great strategy if you know that different Streamable Grid Cells utilize the same Asset Chunk Set. If that is not the case, reuse should generally be avoided.