One of the reasons why "Horo" is considered important is because of the position it occupies in the development of Japanese pop music in the 70's, namely with how it successfully bridges the Western pop and R&B traditions that local songwriters started embracing at the time. Yuming, the originator of New Music, really pushed this trend forward in 1973 when she teamed up with Caramel Mama (who renamed themselves to Tin Pan Alley the following year) to record the founding albums of the genre. In January 1975, Tin Pan Alley would then contribute to Kosaka's "Horo" to bring forward another self-coined genre Wasei R&B (和製R&B). A number of early magazine and radio adverts for the album mention that term, though in the modern days it's referred to as an example of early City Pop, though we would wait another three months before Sugar Babe would fully embrace urban sounds through "Songs". Speaking of Sugar Babe, Tatsuro Yamashita (山下達郎) and Taeko Onuki (大貫妙子) both participated in the backing chorus for "Horo", joined by Minako Yoshida (吉田美奈子), who would also embrace her R&B side later that year with "Minako". And as an icing on a cake, the strings and horn arranger on "Horo" is Akiko Suzuki (鈴木晶子), who debuted in 1976 as Akiko Yano (矢野顕子) when she married Makoto Yano (矢野誠), also a member of this album's personnel.
As for Kosaka himself, he's been an old friend of Haruomi Hosono (細野晴臣) since 1969 when they were band mates in Apryl Fool (エイプリル・フール). Once the band split up and Kosaka went solo in 1971, Hosono would support his music by inviting fellow musicians to play on his albums: Happy End for the first two and Tin Pan Alley for "Horo". Kosaka does look rather intriguing on that cover, a bit like an established middle-class immigrant in 1940's America. "Horo" was one of the last secular releases he put out, as later that year he decided to convert to Christianity after his daughter survived a massive burn. So for much of his career, Kosaka has associated with the Gospel side of things, but because of a revived critical interest in "Horo" over the last decade or so, he recently re-recorded the album in the form of "Horo 2010", this time with blue cover art.