A Mortar Bed is the original "Mud Work" for installing ceramic tiles and although it seems to be a dying craft, it is still done by some tile setters today. The mortar and lath are combined for a thickness of over an inch thick and can be laid out flat over a membrane (slipsheet) which is separate from the subfloor.... any movement of the plywood through expansion and contraction won't effect the tiles.
You have a flat, continuous slab for the tiles to bond to which is the best base in the world for ceramic tiles or stone.
The only down-side is that sometimes many of the renovations can't allow for this much added thickness to the floor so other methods have developed which can still give you the strength and stability, but with less increase in the floor height.
Deck-Mud which is a combination of 4 to 5 parts sand and 1 part portland cement is mixed with only enough water to slightly moisten the compound so it will cling together when compacted. Too much water will cause the bed to crack and warp while curing. The most common application for mortar beds today is when building a Shower-Pan