Why do some give a coin to a traveler?

Question:

Why is it that Jews give a coin to someone who is traveling? Is the traveler supposed to return the coin when they return?

Answer:

Our sages tell us "An emissary to do a mitzvah isn't harmed."1

Traveling is always fraught with danger—though to a considerably lesser extent today than in times past. When we know of a friend or relative who is traveling, we traditionally give them a coin or bill and appoint them as a messenger to give the money to charity in the place that they are going to. In this way, they become an emissary to do a mitzvah (in Hebrew: a "shaliach mitzvah"), and they merit this extra level of G‑d's protection.

Rochel Chein for Chabad.org