Not all European chant was meant to be sung against a drone. A drone is a single note, usually the “base” note of the scale (the "do") or the fifth step up from the base note, held for the duration of the piece or until the mode changes.
Although you could sing Gregorian chant against a drone, it wasn’t really done that way. Hildegard’s chants (she lived from 1098-1179, by the way) are often sung against a drone. A lot of Celtic music (chants included) are performed against a drone.
Something amazing happens when you provide a drone. A drone builds a kind of foundation upon which the other notes depend. Sometimes the drone provides a rudimentary harmony, at other times a dissonance. But there is a kind of homing sensation about it. You feel the tension created by the changing notes and look forward to the resolution when the melody note matches the drone. The same thing continues to hold true in modern music; you wait for the root chord (the base note's chord) of the scale of the piece to know that the piece is finished.
As a singer, performing against a drone provides a real sense of time and space that I have never experienced with any other kind of music. I become more aware of the personal space that I occupy--my own size and shape--and the size and shape of the space or room in which I stand. I am more aware of my breathing, and of how much breath I spend when I sing, of the sounds coming from the room itself, of any sounds outside the room. I slow down, and really get the point of the chant, really feel it in my body, hear from the space when the notes should change to make the meaning clear. I become part of the chant, and the chant becomes part of me.
I had a conductor (David Babbitt of the San Francisco Bach Choir) who said that chant is like a garden hose. Since humans began to vocalize, someone somewhere has chanted. If you take a moment and breathe quietly in the space where you are, you can feel when it’s your turn to come in. Chant is passed along through time and space from person to person, like water through a garden hose. For me, the drone helps my hectic modern mind connect to the hose of history and the future and take my turn.