MMCBRIDE wrote...
Why would I ever need this function?

'Need'? Never. However, IFERROR could be efficient and convenient.
Formulas like

=IF(ISERROR(VLOOKUP(a,Table,n,0)),a&" not found in
table",VLOOKUP(a,Table,n,0))

in Excel 2003 and prior could be rewritten as

=IFERROR(VLOOKUP(a,Table,n,0)),a&" not found in table")

in Excel 2007.