I can’t argue with your experience. My experience with classical musicians is they all know what “natural minor” is because EVERY key signature in Western music is designated by either its Major (ionian) or Minor (aeolian) counterpart. And so do most school kids who paid attention in music class (instead of locking their music teacher in the store room as did some of my class mates once!).
You surprise me because when I was taught (classical) music theory, I had to learn all three - for the somewhat basic reason that all three are used in writing music in a minor key.
The basic rule, as explained to us, was that in melodies you use the harmonic minor when ascending and the natural minor when descending. The harmonic minor was used for forming chords.
Geoff
boy…
this has “upgraded” (degraded?) to something that seems far more technical than what the poster wanted, no?
Granted, some people really care about this stuff at the micro level (and I studied it for years in college), but the poor fellow merely wanted a simple answer.
Let’s face it…99% (90?) of the ITM world hasn’t a clue about this stuff and likely doesn’t care. Can’t blame them. There’s no need for it…until the guitar player asks, “What key?”
Sorry, gents…didn’t mean to burn your thread…but remember to help out those who don’t know and haven’t the time to invest (or the inclination, in most respects).
Why not a separate thread on the technical intricacies of the structure and you can prattle on there to you heart’s content?
No one forms chords with a flute and the notation of the natural 7th of the harmonic minor requires an accidental (ie is a deviation from the major/minor nominated key signature).
Wasn’t drewr asking a question as a flute player?
Getting to the original question. . . . why is the key of the tune important during a performance?
About a year ago I was playing flute for a contra dance in North Carolina. We had all the sets listed out like the professionals we weren’t 
Anyway, we get to a jig set, and I felt pretty confident. Two of the tunes I’d known for years, could play 'em in my sleep. the middle tune was newer, but I’d heard it enough in sessions and practiced enough at home that I felt I could fluff through it decently enough.
Nope!
I launched into the middle tune and it sounded. . . awful. Like, toxically bad, so I started droning instead 
What happened was that the other guys were playing that middle tune in a different, nonstandard key, and I either slept through that part of the rehearsal or they had just assumed I knew what was going on.
There are a good number of tunes that can be played in different keys even on a keyless wooden flute – typically D tunes that can be G tunes or vice-versa. George White’s Favorite leaps to mind. Even the Kesh Jig works in something like four different keys pretty easily.
So, in those cases it’s ALWAYS a good idea to know what version the band will play.
True, but I think you are missing the point I was making which is that any one who has learnt classical music theory should be aware of all the various forms of minor scale.
While you do not form chords with a flute, you do commonly play arpeggios which are still chords but with the notes played consecutively rather than simultaneously.
Whatever instrument you play, it certainly does no harm to be aware of the basics of harmony.
Geoff