Sometimes you need more than 1 output from a sub microflow or you want to adapt an input parameter in a sub microflow withouth having to return the value and adapt overwrite the value in you main microflow.
In this case it would be helpfull to be able to select if the input parameters need to be defined as INPUT only or as INPUT-OUTPUT.
This would make it possible to adjust the value of the parameter in the sub microflow and also have multiple output values.