What is the importance of Shiites?

All muslims have common beliefs :

Belief in Allah:means believing firmly in His existence, Lordship and Divinity, and in His names and attributes.

Belief in the Messengers of Allah: It is to believe that Allah chose the finest amongst mankind to be Messengers whom He sent to His creation with specific legislations; to worship and obey Him, and to establish His religion and His Tawheed (oneness). Allah ordered His Messengers to convey the Message to people, so that they would not have any proof against Him after He sent them.

Belief in the Last Day: It is to believe that the life of this world will come to an end. Allah says:

(Everyone upon it [i.e., the earth] will perish. And there will remain the Face of your Lord, Owner of Majesty and Honor.) (55:26-27)

Every person believes this principals are called muslim.they are common beliefs in islam but after death of prophet differencs emrge about the succor of the prophet.

Acoording the Quran and speeches of prophet, Shia Muslims believe that just as a prophet is appointed by God alone, only God has the prerogative to appoint the successor to his prophet. They believe God chose Ali to be Muhammad's successor, infallible, the first caliph (khalifa, head of state) of Islam. Muhammad, before his death, designated Ali as his successor.Ali was Muhammad's first cousin and closest living male relative as well as his son-in-law, having married Muhammad's daughter Fatimah. So the many belief importance of shia is imams belief which cotinue to present untill twelfth iman (pbuh) .and shia muslim follow iman in everthing .