It's a feature needed only to validate if the configuration makes sense, to be able to complain by logging warnings or possibly refusing to start via runtime exceptions.

In Infinispan for example it's valid to use a RAMDirectory or FSDirectory when using replication on a cache, but when using distribution the index needs to be "clustered".
The Infinispan Directory on the other hand could be used - depending on its configuration - as a performance boosted non-clustered index OR as a clustered index; to know how it has been setup requires access to specific details of its configuration.

If the Directory implementation could expose its capabilities we can build such validation.