What’s that? A true professional tile setter will ‘float’ your bathroom walls, not use cement boards. Often, again in a hurry a homeowner puts extreme pressure on the tiler or maybe the tile person has an impossible schedule and thinks the owner won’t know the difference. After all he gave her a great price, so he must cut costs. I’m here to tell you that floating the walls is a much, much better finish. Let me explain. All home walls are not plumb and straight. When you use concrete boards, screwing them into the walls it follows the shape of the crooked wall. In a hurry the tile guy just screws them in, seals the seam and goes on to laying the tile. Many of the TV shows do it this way which is a huge disservice to the homeowner. Flips are often shortcutted here as well.  The proper sequence is to use shower waterproof paper, a waterproofing membrane. There are many brands at the big box stores.  Next a diamond cut wire stapled on, called metal lath. Following is the scratch coat of mud (concrete) then the brown. The brown coat is the leveling magic. Creating perfectly straight walls where there was none!
This is the professional way of prepping the shower walls. Make sure it is in your tile pro’s contract before you start!