Almost every savoury dish I make at home starts with a bit of oil. Usually olive, but sometimes sesame or canola. While I usually use only a tablespoon or so this oil is vital to my final dinner.
We use fat in cooking for a number of reasons. Firstly, fat can be heated to a higher temperature than water. While water turns to a gas at 100°C, fats remain in liquid form at temperatures above 200°C, which means cooking in fat can give a crisper texture and more intense flavour.
Compare boiled pumpkin, to roasted and you'll see exactly what I'm talking about. Roasted pumpkin has a stronger, sweeter, almost caramelised flavour. It shrinks in size and gains crispy edges. While boiled pumpkin is softer, with a more subtle flavour and watery texture.
In baking, fat lightens bread, helping it rise and increase in volume - it's part of what gives bread a light, fluffy, spongy texture. When making cakes fat helps trap bubbles of air in the mixture, ensuring the cake rises.
When making salads, adding oil to a dressing gives it a more viscous texture, meaning the dressing will coat each element of the salad and evenly distribute flavour.
Basically using fat in cooking intensifies and adds flavour, while smoothing texture and moistening the food you're about to eat.
Photograph by Churl