The Imam is the successor of the Prophet and carries on his work after him. Who appoints the Imam? The Prophet appoints the Imam according to the Divine decree, and entrusts his work to him.
The Imam continues the leading and guiding of the people and clarifies for them Allah's message and commands. The Imam is the leader of the people and the guardian of religion. The Imams call the people to worship of Allah and to do good deeds and help others.
The Imams were the enemies of oppressors and opposed and strived against them. The Imams worked hard to make the people aware of Allah and to establish justice amongst themselves.
If the people have no leader and guide after the Prophet, how can they find the right path of life? The Imam is the leader and guide of the people and the guardian of religion.