Slap bass as used in New Orleans Jazz, "Hillbilly" and "Country" Music in the late 19th Century seems to have been considered a novelty. This may be why the Bass Player was considered a "clown" or "comedian" in many bands. Taking a Bass break was like telling a one-liner joke. The break was also used by the other members of the band to maybe get a quick drink, tune their instrument, or maybe discuss what the next song might be in the set. Sometimes the solo might start out with a non slap solo and evolve into a slap solo by the end. Many times the solo might end with the Bass Player standing on, or riding, or throwing his Bass in the air be fore he was done.
Here is a video of the "Flatt and Scruggs" TV show from the early 1960's that encompasses about everything the "Bass Player" was supposed to be in a Bluegrass Band - Dressed different - "funny", making funny faces, even almost dropping his bass near the end. This is "Cousin Jake"
By the time this TV show spot was shown, Flatt & Scruggs had been playing for over 10 years together. Much had changed in "Country" music during that time. One of the biggest changes was the new "Rockabilly" music that had invaded the airwaves of "Country". This invasion by a young truck driver from Memphis Tennessee, along with his "Bluegrass" slap bass player, and Chet Atkins style guitar player, would forever change music and upright slap bass playing.