Pluggable storage concept enables user to set up different storage engines (SE) on the same node e.g. for performance reasons, each table can be hosted only by one storage.
From DDL point of view SE is specified as part of CREATE TABLE command. But in case of only one SE and some other cases specifying it for each table creates a lot of unnecessary boilerplate code.
To address this and free user from writing exactly the same code a cluster-wide setting defaultStorageEngine should be introduced.