This was fascinating as I know many times businesses actually pay people to line up at stores where they pretend to be customers. When you go by as a bystander you then assume there has to bee something good there for you to check out. This can also help you with marketing such as people taking pictures of the long line ups and posting it online to generate even more buzz.
This one was something I never even thought about though as you have often seen street performers doing various acts where they constantly remind the audience to tip them as that is how they make a living. In this case there was someone advertising for an individual who would act like a regular onlooker where at the end of the act they would tip the performer twenty dollars.
The purpose of this is to influence other people in the crowd to give the person money as well because if you at least see one person doing it then you will likely be more inclined to do it too. Again, I never even thought street performers actually do this to try and get more money from the audience. I suppose they are business too and will use similar techniques like other forms of businesses.