Originally, Bais Din would accept any two witnesses. When minim [heretics] started to try to fool Bais Din, they made a rule that we only accept witness from people who we know are trustworthy.

On Shabbos, if the witnesses were not known to Bais Din, we would send other as witnesses that the witnesses who saw the new moon are trustworthy.

Rosh Hashana 2:2

Originally Bais Din would announce Rosh Chodesh by waving fires on mountain tops. When the Cusites started trying to fool people, Bais Din decided to send out messengers.