The attribute performs the mapping between the work entry and the conn entry. Connectors that operate in Iterator mode and Lookup mode use the conn object to provide values to the work entry. All other modes use the work entry to provide values to the conn entry. In your hooks you will see the entry objects you have available.
In general the work entry is always available and contains the attributes mapped in from the input connectors (e.g. Iterator and Lookup). The conn entry is available after input connectors have read or looked up an entry (e.g. After Get and After Lookup). For output connectors, the conn entry is available when you enter the connector mode specific hooks such as Before Add for Addonly mode.