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):
If set to true then nulls values are removed during attribute resolution.
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.