About the Song
The song Yakety Sax was written by Boots Randolph and James Q. "Spider" Rich in 1963. Randolph recorded the song and released it as a single 45 that same year.
The Benny Hill Show ran first ran from 1955-1968, with Yakety Sax becoming the show's theme song somewhere in between. The song typically was set to "chase" themed sketches heavy with slaptick sight-gags.
In 2002, Something Awful user CheechLizard created a thread entitled "Anything+Benny Hill Theme=Funny." CheechLizard remarked that he came to this realization while listening to the radio, and the thread tended to focus on the layering of audio onto the theme. It should be noted that video remixes were uncommon in 2002.
By 2005, YouTube had become the primary video-sharing site where the lion's share of video-remixing took place. And so, Yakkety Sax can be found there as well.
Anything Better w/ Benny Hill Hypothesis
The titles of many videos suggest that nearly anything is better when paired with "The Benny Hill Theme."