Imagine you have a reasonably complex Data Source configured that uses Dynamic Columns and possibly Lookups to define your Source Data Transformations. Currently if you wanted to use this again in another Sync job then you would have to copy it all again which isn't ideal.
But now … with the new Data Sync Project File provider you can reference an external Data Sync Project as a Data Source.
So this Source appears just like a normal Data Source where you can then create new Dynamic Columns or Lookups against the already Dynamic Source.
You can see from this that you could easily define a set of Data Sources in Data Sync Project Files and then simply attach them to new projects.
This new feature is in Data Sync Release 3.0.560