Why did Allah send the Messengers?
To call mankind to His worship and to reject the worship of anything besides Allah.
Evidence from the Noble Qur'an
{And verily We have sent among every nation a Messenger (proclaiming) worship Allaah alone and avoid Taaghoot (false deities)}, {Soorah an-Nahl, verse 36]
Evidence from the authentic Sunnah
((The prophets are brothers and their faith is one)), [Transmitted by al-Bukhaaree and Muslim]