Why did Allah send His Messengers?

Allah sent His messengers in order for Him to be worshiped alone without any partner associated with him, as He said:

“And we did not send any messenger before you (O Muhammad (PBUH) but we revealed to him (saying):{none has the right to be worshiped but I (Allah}, so worship Me (alone & none else).” {Al-Anbiyaa (21):25}

There is also the Prophet’s saying: “I am sent with the sword when the day of resurrection is around the corner, until Allah alone is worshipped, & there is no partner for Him.” Reported by Ahmad.