why are the best tasks ones that have emergent properties?

According to the Oxford dictionary of English, an "emergent property" arises "as an effect of complex causes and not analysable simply as the sum of their effects".

The best tasks have emergent properties: the task is designed so that each student puts a little creativity into it and suddenly magic happens!

Compare the two following tasks as an example:

1. The customer has to order food from a waiter. There is nothing creative here. The student playing the customer simply recites the names of foods and the one playing the waiter repeats the phrase he learned for accepting the order. No creativity, no magic, no fun.

2. The customer has to order food from a waiter. Same task, but what if the waiter has a secret list of foods that appear on the customer's menu that the kitchen doesn't have?  Or if the waiter is secretly told to refuse to serve any foods starting with the letters H, S or F? Or if both characters are given secret cards describing their mood and they have to work evidence of these moods into the scene?  It’s still essentially the same food-vocabulary related task, but now it becomes much more interesting.  All you have to do is sneak in an obstacle, a difficulty, a challenge.  More creative, more magical, more fun!