Shortens gluten strands and makes baked products more tender Plastic fat allows incorporation of air. Spreads and coats to assist in making baked products more flaky. It actually shortens gluten strands. Fat also gives richness and flavor and helps in browning. To shorten effectively, fat must have the capacity to coat and spread well and adhere to flour particles.
Oil is liquid at room temperature.
Oil does not have capacity to incorporate air so may give too much shortening and cause no gluten to develop. Don't substitute oil or melted butter for plastic fats. Therefore you should not substitute exactly in recipes. The proteins in eggs will coagulate and that helps in the rigidity of air cells. Eggs also give flavor and richness.