Why use lard in tortilla dough?
Rendered pork fat—manteca de cerdo—is our preferred cooking fat
  throughout this book. Lard has a relatively high smoke point so it is
  ideal for frying, and it provides a richness, dimension, and
  distinctly Mexican flavor that you just can't replicate with using
  other fats...Vegetable oil is a fine substitute


However specific recipes may really need lard:


  Note that the flaky texture and slight richness that make flour tortillas so compelling are derived from lard. I've found that vegetable shortening is a poor substitute here...


Source: Alex Stupak and Jordana Rothman. "Tacos: Recipes and Provocations".