It's always useful to know what pitches occur in a tune and what its tonal centre is. For the heptatonic modes we think of as coming from chant (they don't, exactly) , that can be learned very quickly, as Don says.

The gapped modes (specific to folk music, you don't get them in mediaeval chant) give you a *lot* more information, which is particularly useful for players of diatonic instruments. They take a bit longer to understand, but for something that's getting more results you'd expect that.

"In the Major, the dominant is always the 5th, but that wasn't always true for Ionian. "

Example? When did anyone ever do chant in Ionian?