If you don't add that and the field is null then it will error out. If you use the Len(FieldName & "") = 0 then it takes into account both nulls and zero-length strings. In this case, since it is a single, it could probably get by with just
FieldName Is Null
as you can't put an empty string into a number field. But it won't hurt if you use it and if used that way all of the time it is a consistent way of doing it so you don't have to worry about what to choose to write for which data type.