Early american theater of the 1800's and into Vaudeville of the early 20th century had songs with little or no relaton to plot.
Initial film musicals of the early 20th century were a series of songs, with a vague plot included. (See "Sunnyside Up " 1929)
Broadway musicals of the 1950's evolved to have the musical numbers as vehicles for "plot development", ie a character or situation changes by the end of the song.
Television variety shows of the 1960's and 1970's ("Laugh In " '68-'73 and the "Carol Burnett Show" '67-'78) reverted to the independent-song format, dispensing with the idea of plot entirely.
The Monkees included a series of romps, which were clips cut to songs, which sometimes were relevant to the plot or were straight "music performance" pieces. They were brought in as an aspect of the "marketing" facet of the show, and ememplified the hybrid of two industries: television and musiic.
The last 3 episodes of the series each included a guest musician in an interview/performance format. Charlie Smalls ("The Wiz "), Frank Zappa and Tim Buckley were chosen by Davy, Mike and Micky respectively for their guest spot. Peter's choice was Pete Seeger , but his schedule did not work out in time for it to be included before the end of the 2nd Season.-link to Seeger's Music
It has been speculated that If there had been a 3rd Season, the format of the series would have evolved into a Variety Show Format: interviews, skits and song performances.