Tile that is not effectively backed can crack and degrade over time. Even a few hairline cracks can allow water infiltration, leading to mold and mildew. Thus, a solid base is needed to inhibit tile movement.
Not only that, you need to have the right type of base for the tile thinset to properly adhere. Backerboard is porous and allows the thinset to "grab" on.
Being an inorganic material, tile backerboard will not decompose in wet areas. This is one reason why backerboard is the favored method of backing tile in showers and bathtubs. Plywood and ordinary drywall will decompose in these environments (even though drywall is made of gypsum, an inorganic material, it is faced with paper).