The Simple attribute definition copies input attributes to an output attribute.
Historically this was used to 'expose' attributes sourced from a DataConnector, turning them from internal data into "real" attribute objects. This is no longer required as a DataConnector can "export" its results to produce first-order attribute objects if they require no post-processing.
Remaining use cases for this definition include (not exhaustively):
In this example, the eduPersonEntitlement attribute is produced by merging two sources of entitlements to get the final set of values. If there were only a single source of values, the definiton would likely be unneeded.