Please enlighten us. If a tune is composed in C major versus C Ionian what are some of the important differences.
Are you familiar with the functional names of scale degrees? E.g., Tonic, Supertonic, Mediant, Subdominant, Dominant, Submediant, Leading Tone...
And are you familiar with musical scenarios that ascribe those same names to the diatonic chords built on those scale degrees? So that, for example, the IV chord in the key of C major is referred to as the Subdominant chord?
Those musical scenarios all presuppose that the scale degrees, and the diatonic chords built on those scale degrees, will
behave in a predictable fashion....because those names and the qualities they describe refer to how pitch collections
function traditionally/historically.
In modal music those names and functions are effectively meaningless.
You can create a chord progression in C Ionian where the simultaneous sounding of the pitch classes G-B-D-F doesn't "want" [sic] to "resolve" [sic] towards C, because the concept of a dominant>tonic resolution is entirely the province of the common practice major-minor tonal system. Modality allows the musician to explore pitch space without the same implications of inherent meaning that tonality assigns to collections. That's
why we have two separate names for what is architecturally the same object: to differentiate between subsets of that object that have ascribed behaviors and tendencies, and subsets that, while made of the same constituent parts, are implicitly free of those same behaviors and tendencies.
Whether one
hears them as different objects is a whole 'nother issue entirely.