Ginsberg says part of it was practical--a way to "recycle" unsold bread. He says that some countries have laws limiting the amount of old bread that bakers can use in dough. He does say that "chemically the gelatinized starches that make up the bulk of stale bread bind water far more effectively than raw flour, tightening the crumb of the finished loaves and reducing the notorious tendency of some rye breads to crumple at the slightest touch" (p. 46). He seems to suggest that any type of bread can be used, since the amount (no more than 10%) is small enough not to affect taste.
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