What is the purpose of sending the prophets? Is it not obligatory to send them?

Answer

In the name of Allah, Allah (the Exalted) has created humankind so that they win the grace of the Hereafter through the good behaviour in this world. Since humankind themselves cannot know what they should or should not do, it is necessary that Allah sends the prophets to guide people. This is kind of Allah Almighty. Allah is the all Guide.