Summary: Fun is important to design usability (Monk et al 2002; Overbeeke et. al. 2002; Hazzenzahl and Burmester 2001). Fun activates several important cognitive processes, known for centuries from games we have created and played. Think of a fun game you lasted played...These same processes make fun an important design usability tool.
First, fun loosens structured cognition making it easier to learn. Fun engages whole-brain learning by engaging our right-brain processes (in the right prefrontal cortex).