There is a difference between Nabi and Rasul:
The Rasul received the complete revealed books. But the Nai did not receive the same. They used to preach religion following the book of their previous Rasul.
Necessity of sending Prophets:
- To preach acquaintance of Allah: the Prophet manifested before us the acquaintance with Allah.
- To guide to the right Path: The right and wrong paths were not known to us. The Prophets guided us to the right path.
To teach the rules of Ibadat: The Prophets themselves practiced the commandments of allah and then trained the people to follow them.
Rectification of character: The Prophets possessed high character. They rectified the character of the people.