Fat brings control, stability, consistency and so much more to taste. It becomes a container for all of the individual ingredients and tastes to converge. It can also allow ingredients and tastes to stay near each other without merging.
Sometimes you want individual ingredients to stay as they are and hold their particular flavor vs. all coming together to form a new one, it really depends on what your goal is. If you are making a roasted garlic and butternut squash soup then you may want to taste both ingredients very clearly. You immediately get a blast of sweet butternut squash while the roasted garlic slowly takes over the background. As it all washes down you are left with the subtle onset of nutmeg and sage.
This kind of time released, compartmentalized flavor is created…it is concocted, and fat is a very effective tool in doing so. Fat buys you time and space in the mouth. It’s like fat makes the mouth twenty times bigger than it actually is and you can give your tastes a running start if you want. You can set one up in the far left corner and one up in the far right corner so that they only come together when they’re being chewed.