The NVL function is used when you may get a null value and want to return something else.  In the example below, VENDOR_SUPPLY_NAME is a char field with no nulls allowed, however doing the TRANSLATE function can return nulls so I use the NVL function to return ' ' when a null is returned by the TRANSLATE function.
select NVL(TRANSLATE(A.VENDOR_SUPPLY_NAME,
             '/-.,''''#' ,'  '),' ')
from tidvnmst
where .....