Imagine that you have made lots of code, wiring up several VI etc. all with the same data, a cluster for example, going in and out. Now you regret not having defined that wire as a typedef. Today replacing the data with a typedef will involve a lot of steps; you start by clicking on one of the controls or indicators, and create a typedef...then you need to do the tedious work of replacing all the other controls and indicators up- and downstream.
Would it not be nice if you could just right-click on the wire and select "Define type"/"Create type definition" or "Replace with type def/class" - and then choose to have the type definition automatically replace everything along the wire ("propagating type def")?
This idea was inspired by and first came about as a comment to this idea by cowen71.