Magic Tallow is an early "hidden" research that is used in a few other more advanced crafting recipes. When you unlock this research you also learn how to create Candle, which are basically just a better looking torch when crafted into a Magical Building Blocks, although one that can only be placed on a flat surface, not hung on a wall.
If you have an alembic, then Magic Tallow becomes an inexpensive way of creating Praecantatio. Simply place a magic tallow in your crucible and right-click with your wand. You'll get your tallow back, with a profit of one essentia of praecantatio.