Why backerboard for slab????

Why backerboard for slab???? Images

Why backerboard for slab????

Why backerboard for slab???? Description

I read a post from msmrbl, where she stated that backer board was better than plywood for 20mm slab (3/4). I've been installing for several years and (http://maplered.com) NEVER have I heard of, or seen backerboard used instead of plywood when installing slab. Am I missing something? Please let me know what thats all about.
thanks
mike

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