People enjoy music for different reasons. Some people want to be soothed and some people want to be charged up. Some people love complexity and sophistication; some are more into the raw power. Many people are only able to appreciate one specific type of music. They aren't really music fans as such, they're jazz fans or classical fans or rock fans.
Personally, I find that different types of music speak to me in different moods. I like punk rock when I want a jolt of pure energy, traditional Celtic music when I want to immerse myself in a culture with soul, EBM or futurepop when I'm feeling a little philosophical, crazy avant-garde art-rock like Laibach or Devil Doll when I'm feeling crazy and artsy, and meditative music like New Age or post-rock when I'm meditating or relaxing.
“Mono” has the ability to speak to more than one mood at once, because it isn't just soothing like “Enya” or just sweeping and epic like a lot of heavy rock bands. It combines the epic quality of great rock with the meditative beauty of New Age music. You could say that it's the equivalent of New Age music for lifelong rock fans.