When creating complex UDT's there is no way of knowing or seeing what UDT's go into building other UDT's or there relationships.

Being able to design your UDT's within an UML Application would give you a visual picture of how things go together and their Member data types (documentation) and being able to import the model into Ignition to create the UDT Types cuts down on errors and typing.

Once in Ignition you would still have your UML Diagram (App/Printed) to help with making edits.