I agree it's important to know what note to start on, but that doesn't mean you need to to know its name. Many experienced singers have enough sense of their own vocal range to be able to internalise this and pitch a song correctly. Besides, many folk songs don't have a challenging range so most singers should be able to cope even if they are slightly out.

As Tattie Bogle says, some singers will start in the wrong key and have to stop and start again. Those singers would certainly benefit from using a reference note to help them pitch it correctly. However don't overlook all those singers who are able to pitch it correctly.

It depends on how you think about music. Not everyone has a musical education and many don't think in terms of keys and named notes. I learned to play several instruments by ear well enough to perform semi-professionally. I may know the key (diatonic instruments are limited that way) but I usually don't know the name of the note associated with a button or fret. I think of chords as finger patterns rather than as notes. Ask me to play an F# and I'll have to work up the scale until I get to it, but when I'm playing a tune I know where to find the sound that is called F#. I'm even less conscious of key when I'm singing unaccompanied, but I know where to pitch a song to suit my voice, and I'm aware of those songs with unexpected ranges which may need to be pitched more carefully and which may need a reference note.

The point I am trying to make, somewhat laboriously, is that whilst Sandman's advice is sound, people should not be intimidated into thinking they must understand keys and notes in order to sing.