No man is an island, the world we live in is vast with countless people sharing countless memories and experiences with one another. We have troubles, we have doubts, and we also all believe in ourselves to a vast variety of extents.
Self-esteem is usually something you build up (unfortunately sometimes destroy) on your own. But friends also have quite profound effects on your self-esteem, on how you value yourself.
A friend will believe in you and what you are capable of. He/she values you even if you yourself believe you are not that important. A friend will support you, be with you through thick and thin and believe in you every step of the way. Your friend will complement you, acknowledge you, keep you going when you feel that you can't go through anymore.