Imagine trapping a bank statement where some entries have a value in the Credit column, some in the Debits, but none in both. You can use the numeric or trap to select those rows which have a number in either column. If you tried to use the N numeric trap, defining this in both columns would result in trapping no rows.
It's also very helpful when dealing with PDFs where the data might not line up so nicely.
Hope this helps,