At stand up shows, people who film usually put it up on the internet which reduces the originality of the artists. Other struggling comedians usually use jokes from such sources without giving a credit. Moreover, it also allows the stand-up comedians to use the same jokes over again at different instances, especially when close performances are scheduled.
This is because coming up with three original scripts for three shows scheduled one after another is almost certainly impossible, if the quality of content has to be kept high. This is the prime reason why cameras are not allowed in stand up shows.