Flour is not only used in cooking and baking but in other processes, as well. The Gemara on our daf discusses a case where flour is used in preparation of leather or other products made from animal skins.
The Sages taught [in a baraita]: With regard to tanners’ bowls into which one placed flour in the production process of leather, if the flour was placed within three days of the start of Passover, one is obligated to remove it, as it is still considered edible leaven. However if one added the flour prior to three days before Passover, one is not obligated to remove the contents of the bowl, as the flour will have already been rendered inedible by the odor of the vessel before the beginning of Passover, and is no longer considered edible.
Rabbi Natan comments on the baraita, saying that the three day rule is only true if animal skins had not been put into the trough. If, however, the process of preparing the hides had already begun, then there is no longer any need to clean the trough, since the smell of the skins would make it impossible to eat the flour. Rava rules like Rabbi Natan, arguing that even if the hides were put in a short time before Pesah, the flour is already considered inedible and there is no need to destroy it.
With regard to the process of tanning leather, there were a number of different methods used in the time of the Mishna and Talmud, depending on the types of skins and the desired end-product. Generally speaking, flour was one of the ingredients used when preparing skins for use as parchment. The leavening process itself played a role in transforming the skin into that material. Tanning with flour was only the beginning of the process, and it still needed to be worked on further before it was ready to use.