Using variable-length fields for temporary variables can improve the performance of string operations, as well as making your code easier to read since you do not have to save the current length of the field in another variable for %SUBST, or use %TRIM to ignore the extra blanks.
If a subprocedure is meant to handle string data of different lengths, using variable-length fields for parameters and return values of prototyped procedures can enhance both the performance and readability of your calls and your procedures. You will not need to pass any length parameters or use CEEDOD within your subrocedure to get the actual length of the parameter.