Append Variable Activity in ADF & Synapse | Add a value to an existing array variable in ADF

Use the Append Variable activity to add a value to an existing array variable defined in a Data Factory or Synapse Analytics pipeline.
Note

The appended variable value does not appear in debug output unless you use a Set Variable activity to explicitly set a new variable with its value.

[cherukurid]

Why we have to take default value to the variable, generally default value has to be taken when there is no input given right but here it is taking both values ?