Modes as Keys

If you look at the major scale, it can be thought of as a key. Traditionally a key was an aide to musical notation which put all the notes in context, or the sake of clarity.

If you wrote a piece of music in Gb it'd contain the notes Gb Ab Bb Cb Db Eb F, without a key signature at the beginning of the stave, you'd need to show each and every instance of the flat notes in the scale, it'd make the reading very difficult. So you have a key signature and that says every time there's a note at that pitch it's got to be sharpened or flattened to match the notes in the key signature... now you can read the musical notation with greater ease.

Now the thing is that a lot of music now contains only chords that are diatonic (of the scale), if you harmonize the major scale you get these chords, this example using C major:

C major 7th
D minor 7th
E minor 7th
F major 7th
G dominant 7th
A minor 7th
B minor 7th flat 5th

By harmonizing I mean take every other note of the scale so CEGB, DFAC, EGBD, FACE, etc.

So modes aren't that different from scales. I mean essentially all a scale is, is a pattern or intervals, like TTSTTTS and a mode is a rotation of that pattern so TSTTTST etc. When you look at melodic or harmonic minor then the patterns become important and not the start or finish points, because the angularity of the pattern affects the chord choice and the sound of progressions in a really fundamental way because the chords are different.

Modes as keys:

I II III IV V VI VII
C Lydian CMaj DDom Emin F#m7b5 GMaj Amin Bmin
C Ionian CMaj Dmin Emin FMaj GDom Amin Bm7b5
C Mi