Originally Posted by firmbeliever
It established the messenger/s as a truly chosen person by the Almighty.
It helps people to discern truth from falsehood.
Sometimes the very people to whom messengers are sent demands miracles as proof of the divine connection.Some miracles happen while the messenger is alive amongst his people,while other miracles (prophesied in the revealed texts)takes place after the messenger has passed on from this world.
Prophecies are important in that people in later generations who may not have lived the generation of the messengers may learn from the same revealed texts and understand the divine source of the message.
Directly from the Almighty to the messenger or through Angel messengers (Gabriel/Jibrael-alaihi salaam).
And these messages are in turn either written laws or additions to already given laws through previous messengers.
Just my personal thoughts on it.