Why is NASA conducting a human Mars mission on Earth?

NASA recently sent 4 humans on a Mar's mission known as Crew Health and Performance Exploration Analog to study the impact the Mars habitat will have on humans. It is an year long mission, where the four humans will be conducting various activities such as spacewalks, cop growth, maintenance to collect performance data that will assist NASA in making decisions to plan and design a successful human mission to Mars.

But there is a twist, it's actually conducted on Earth in a 3D printed facility in Houston.