She may be the first composer who has a known biography, so that makes her remarkable just for the fact that she is the first composer that we know a good deal about. She wrote music in the "plainchant" style, which is the Gregorian Chant style. Gregorian chant - attributed to Pope Gregory I (540-604). However, that was actually what is called old Roman Chant. Roman popes started getting Gregorian chants from Germany (where Hildegard von Bingen was from) around 1014. Hildegard was probably born in 1098, so she was in the right country as far as Gregorian chant was concerned.
Hildegard was a polymath: a physician, poet, scientist, and many other things, as well as a writer, when few women were writing or even allowed to have a voice, especially in religious interpretation. I read that Pope Eugenius actually encouraged her to write, after learning of her divine visions which influenced her writing and music. Her plays were performed in her convent, and she was beatified and is often referred to as a saint. I can't speak to how much direct influence she had on Gregorian Chant with melodies/harmonies/style, but given these facts (chants started being exported to Rome during her lifetime), her talent for writing music and religious texts, and her revered reputation which prompted people to seek her out for advice (especially for a woman in this time period), she probably had at least some general influence. Her plainchants were mostly monophonic - one melody line. Chants eventually added a harmony (polyphony - more than one melody) which was called "organum" in the late 9th century. So, my guess is that, at the least, she contributed to the tradition of monophonic lines.
Check the enotes link below: it says her melodic lines were unique and unlike standard Gregorian.