Why is it so important to be modest? What is the external part of our modesty, and what is our inner modesty? The external kind of modesty is well-known – to dress modestly. But what is the inner meaning of modesty?
The answer to this is that modesty is part of the purpose of Creation, which is to reveal Hashem in the world. Hashem is modest, because He is hidden from the human eye. So must we be modest, because when we act modest we resemble Hashem, who is modest and hidden.
This reveals Hashem in the world. When we are modest, we keep ourselves hidden, which shows people that there is a Hashem in the world – and when we do this, we fulfill the purpose of Creation, because we are revealing Hashem onto the world.
[for an in-depth explanation of modesty see here]