Why did Allah send Prophets?

Allah has created the human beings with an innate awareness of right and wrong. Allah says in the Qur’an:

And [by] the soul and He who proportioned it

And inspired it [with discernment of] its wickedness (corruption) and its righteousness (piety)[1],

 Technically speaking, this in-born consciousness of Allah and sense of righteousness and corruption should be enough for a person to live a righteous life. However, Allah knows that human beings are weak, they tend to follow their emotions and desires, and that there would be forces in the world that He permitted to exist, which would sway most human beings from the correct part and lead them into corruption. So, on the basis of His mercy, His love for mankind, His generosity and kindness, He didn’t leave them alone to find their way. He sent Prophets to humankind. He revealed His will in the form of scriptures and the Prophets that He sent would show the people how that righteous life should be lived, and how that will of Allah should be implemented in all aspects of their lives. So, Allah chose from amongst mankind certain individuals whom He blessed with revelation and responsibility of conveying that revelation to those around them. Allah chose them.

Reference:

[1] Surah As-Shams, 91:7-8