Using the typical paper tape and mud used in drywall applications won’t work. Neither of these are water resistant and as such are unsuitable for wet areas. Instead, the use of an alkaline-resistant mesh tape and thinset mortar is recommended. Both are water resistant, compatible with the cement board, and add to the durability of the system.
Why bother to use mesh tape and thinset to mud all the cement board joints if everything gets tiled anyway?
The taping and mudding bonds the individual cement boards together to create a monolithic face. In other words, the mesh tape and thinset help to cut down on movement between the boards, which is crucial to prevent cracking in the grout lines and wall tiles.