There's more than one way in which travel contributes to building confidence. The most obvious is that you have to step outside your normal day to day routines and familiar places and people to travel at all. Following on from that is the fact that travel opens your eyes to new things. Your senses are bombarded with new smells, sights, sounds, tastes and sensations; it provides you with more interests and more to think about.
Travel expands your mind and makes you more interested in the world outside yourself, which in turn makes you a more interesting person. How could this process not build your confidence, Do you remember the first time when you travelled somewhere new without being in the company of your parents,
Can you remember the feelings which were elicited in you, And do you recall how you felt when you came home and you had so much to talk about and tell everyone about, Nearly everyone in this situation experiences excitement and a natural high. You no longer have time to worry about what to say and how to say it; you have so much new "stuff" going on in your head that it just pours out naturally; conversation flows naturally. You feel comfortable because you know you have a lot of interesting things to say.
This makes you feel a whole lot more confident. I was chatting to a young friend the other day who had just returned from a month in Egypt; she was animated and excited and she was wearing a great big confident smile which spread from ear to ear. She had so much to say! Pyramids, museums, animals, quad-biking, camels - you name it, she did it! It was great to see her and to listen to her. I'm not saying that she was not confident before she travelled; I'm merely saying that she appeared more confident.
The fact that travel builds confidence is really a very obvious fact. I know it's not one's primary intent when deciding to go on a trip or on a holiday. But it is definitely the very greatest benefit which one gains, even though in effect it is merely a by-product of the process of travelling. If you think about it, your body and mind are intrinsically linked. It is a known fact that as you move your limbs by walking or running (or any other form of exercise) your brain reflects a simultaneous instinctive reaction of moving and stretching.