Most music theory training centers around intervals to some degree. We use it to describe the pattern of sequences (scales, motifs, melodies), the distance between stacked notes between voices or in a chord, to describe the consonance/dissonance of a given set, and the relationships between different sections in different keys. The reason for this importance being that when we experience music it's not just notes in isolation, but within the context of other notes. The way we discuss this experience is via intervals. Even if not being done explicitly, a listener may describe a feeling from a particular part of a song that was the result of how notes interacted (the interval content).