See also the previous articles: Interesting Tricks and Interesting Juggling Tricks
I missed (at least) one reason why tricks can be interesting.
I call this reason: The Toy Factor
Sometimes you see a performance and they introduce or remind you of a new concept or a new prop, and you just wish that you could play with it
This is very similar to child behavior with toys: Seeing other people play with something makes you want to play with it too. The feeling is located around jealousy or curiosity.
The Toy Factor might not be so common for a regular audience, but I don't know this for sure. It is of course mostly trained jugglers that see the thousands of new possibilities that arrive with a new thing, though perhaps non jugglers would have a similar reaction to something less complex but still new to them.