From Barry Mills’ Zappa: A Biography:
[Zappa’s record label] Bizarre did a deal with Warner/Seven Arts to distribute their records on Reprise. …Now all they needed was some ‘product’ to release. …Warners must have had severe misgivings about their involvement with Bizarre. The distribution deal meant that Warners had to pay Zappa each time he delivered some product, but some people at Warners thought he was abusing the terms of the agreement by coming up with any old thing to release. Zappa had known about Wild Man Fischer for some time (there is even a brief quote from one of his songs, ‘Merry-Go-Round’, on Lumpy Gravy.
Live street recordings appealed to Zappa, and An Evening With Wild Man Fischer can be seen as an extension of his use of voice collages on the Mothers albums. Wild Man Fischer became the first release on the new Bizarre label when the single ‘The Circle’ came out in October 1968.
Postscript, from the Wiki:
Zappa was responsible for Fischer’s initial foray into the business of music, an album called An Evening with Wild Man Fischer, contains 36 tracks of “something not exactly musical.” Zappa and Fischer remained close — until Fischer threw a jar at Zappa’s daughter Moon Unit Zappa, barely missing her. Due to this falling out, Zappa’s widow Gail Zappa has chosen to not release An Evening with Wild Man Fischer on CD, to the ire of the small but dedicated Wild Man Fischer fan club.