The underlying assumption of an education system that relies so heavily on test-based assessment is that content is what matters.
For those who prioritize learning that can be measured using only quantitative assessments, game-based learning probably just looks like a way to increase student engagement and content retention. It might seem like a complex workbook, or an entertaining quiz. Perhaps game-based learning looks like a great tool for practice and drilling, like a super sophisticated flash-card system that makes memorization more fun. But this kind of thinking doesn’t take into account the broader understanding of what matters. Game-based learning is a great classroom tool because it allows for interdisciplinary learning through contextualized critical thinking and problem solving.
Games in the classroom can encourage students to understand subject matter in context — as part of a system. In contrast to memorization, drilling, and quizzing, which is often criticized because it focuses on facts in isolation, games force players to interact with problems in ways that take relationships into account. The content becomes useful insofar as it plays a part in a larger multi-modal system.

Definition
Game-based learning is an instructional method that allows students to experience, understand, and solve problems in the world of a particular subject, or system, from the inside.

One promise of game-based learning is that it has the potential of building comprehension and literacy rather than retention. It does this by combining instruction, practice, and assessment. Teachers become the facilitators of a process where instruction is experiential. Practice is project based, requiring students to solve new problems and address new challenges using the new ideas to which they’ve been introduced. And assessment no longer measures a student’s ability to regurgitate information, or to choose among multiple answers, but rather, to use the content, or subject matter, in context. Even more impressive is that in order to successfully manipulate one piece within a comprehensive and complex system, the students must understand every piece of the system.
http://blogs.kqed.org/mindshift/tag/games/