Nonessential Equipment and Furnishings (NEF). NEFs are those items installed on the aircraft as part of the original type certification (TC), STC, engineering order, or other form of alteration that have no effect on the safe operation of flight and would not be required by the applicable certification rules or operational rules. They are those items that, if inoperative, damaged, or missing, have no effect on the aircraft’s ability to be operated safely under all operational conditions. NEF items are not instrument and equipment items already identified in the MEL or CDL of the applicable aircraft. They do not include instrument and equipment items that are functionally required to meet the certification rule or for compliance with any operational rule.
(Taken from FAA MMEL PL-25)
I'm going to state that NEF's exist because common sense does not. And then duck.
It shouldn't be necessary to state that a passengers foot rest isn't required for flight, and common sense wouldn't even have anyone ask "can I fly if this is broken and yet isn't on the MMEL?" - the number of these required for flight is nil, and the MMEL doesn't as a general rule list all the "nil required" items. But, people kept getting caught up on the "undefined items", so now they get listed.
For similar (but opposite) reasons, I've seen it explicitly written that although MMELs do not contain the "wing" as "required for flight" this doesn't constitute permission to attempt to fly without a wing....