Conversions in prep recipes allow you to make a prep recipe by weight but serve it by volume and vice versa.
It's also great for pies, trays of baked goods or breads where you make a whole and then slice it into serving amounts.
Here is an example of a vegetable side dish which cooks down to 8 lbs, but is served by the 1/2 cup.
The Yield/Portion is 8 lbs, and below that, in Conversions you'll see that there are 4 cups per lb.
And here we have the conversion of 4 cups per lb. Now in the recipe we can use it by either lb, oz or any volume (tsp, tbl, cup, pint, gallon)