Mucilages are useful in medicine as vehicles for various insoluble and other drugs, and in the arts as thickeners (in calico-printing, dyeing, &c.).
Thickeners, like arrowroot powder or cornstarch, are substitutes for recipes calling for gelatin.
Starches are the most common form of thickeners for sauces.
There are other starch based thickeners available to cooks like cornstarch, arrowroot, or potato starch and even rice flour, but generally all kitchens will have flour and butter and these are the best ingredients to use to make a roux.
Xanthan gum and carageenan are fairly common thickeners used in ice creams and frozen yogurts.