Corn starch was used for various purposes, including for laundry. They were buying boxes of it and just chomping down on it.
A little corn starch is good for thickening soup or desserts, or to put on itchy skin, or to use for various other things. However it's not normal for people to eat entire boxes of it at one time.
I was laughing at the part that said the negro lady had a "powerful desire" for more corn starch. It makes me think of those old tv shows where some negro says "Oh I gots a powah-ful des-eye-ah fo' dat"
The only thing I can think of is that maybe, because they were poor, there was a nutritional deficiency, and their body craved whatever mineral they were missing. Culturally, the response to this was eating clays, which provided the missing nutrients in the diet. Without access to the proper kind of clays, they used corn starch, which could have a similar texture, and was inexpensive, as a substitute.