He came as a witness, to bear witness about the light, that all might believe through him. (John 1:7, ESV)We have learned that in Jesus are life and light. What is marvelous is that God also sends people to testify to that light. God could put huge signs in the sky, and sometimes He does send us signs, but so often He sends people to be witnesses.
Though I could not even begin to fathom the mind of God, I do believe that at least one reason why God sends people is because it increases the role of belief in the life of His followers. If there were huge signs in the sky, it would be more difficult to deny.
The word translated "witness" is the same word where we get "martyr." You see, these witnesses give of their lives (some literally) for the message, for the testimony. Faith takes a dominant role as others believe the message of those who are sent.