Usability testing gains it's strength from direct observation. Like listening to someone describe water vs. putting your hands under a tap to directly experience water, direct live observation of a usability test is more "real". Moreover, direct observation is critical for accelerating understanding and decision-making.
Attending and getting as much live Usability Testing exposure as possible is crucial to making better decisions, understanding the context of user problems or issues and for strengthening your user empathy and advocacy skills.
Source: Usability Testing Central: What makes live observation of a usability test a must?
Frank Spillers' Comment:
A live observation of Usability testing offers an unprecedented opportunity to understand your users. Ultimately good usability skills come from being able to "think like your users". Usability testing observation helps you build up the correct patterns and generalizations, and over time, this can ultimately make you smarter about what makes for good usability and what doesn't, in your design efforts.