A good fit is one that doesn't just slide onto the body without discomfort — it's one that's tailored to flatter your frame, emphasizing the features you want to draw attention to and minimizing the ones you want to conceal.
Clothing should act as a tool to enhance your ability to achieve your immediate goals. How can this fail? Say you've overdressed for a casual networking lunch. You might look impeccable, but it's likely that fewer people will approach you and many might find you pretentious.
The fabric used can affect many things about a garment, including its weight and texture – but also the way it drapes on the body, the way it feels after hours of prolonged wear, and the care it requires.