When you can directly meet the Prophet (SAWA) or Ahlul Bayt ( AS), you will not need any one after them, but when we are unable to meet them, we need to follow their teachings and since the gap between us and them is big, there are many different narrations on same subjects, we need to refer to the most learned  scholars who know which narrations are authentic and which narrations are not authentic, and can explain the meanings of the narrations in the best way. That is why, you need to follow the evidence provided by the Marja' of Taqleed to be sure that you are really following the Prophet (SAWA) and Ahlul Bayt ( AS) and not something or someone else.

With out following the Marja' of Taqleed, your religious acts will be with out authentic evidence. Such evidence can not be found and taken  but from the most learned scholar who is called Marja' of Taqleed.

Wassalam.