The great value of cement board is does not rot, warp, grow mold, or deteriorate, when subjected to water. Wood obviously is not a great material to use in wet applications, and even green board, a more hardy type of drywall, is not recommended for places with constant water usage, such as showers. Cement board is mainly used as a sub-surface for tiling.
For jobs that requires a product that gathers fire resistant, moisture and mold resistant plus durability and rigid characteristic as cement, fiber cement boards is your choice. For exterior and interior applications.