More nostalgia and looking back to the beginnings of what became known as 'street corner symphonies' when youngsters would sing a-capella harmonies on the streets of their neighbourhood. The beginning of the 'doowop' style of music.
Because it was a-capella the doo wop and nonsense lyrics acted as 'fills' replacing musical instruments thus helping the melodic flow of the music. Instruments/orchestrations were added when these groups made it into the recording studios but without sacrificing the added colour of the doo wop lyricism. It is how musical styles develop and change over the years and this new style was a progression from previous years.
The style probably began with the very popular vocal harmony group The Ink Spots in the 1930s. The first video below is 'I miss you so' by Cats and the Fiddle which could easily be mistaken for a 1950s record and the rest, as they say, is history. and briefly encapsulated in the other videos, roughly in chronological order.