Ok, let's try to explain this concept I recently learnt.
Let's first think about a man in the jungle...without anyone to teach him anything. Considering his surroundings and what he can see, this man would indeed adopt the ways of the animals he sees. If a man on the other hand were educated, he could bring about all the qualities and capacities that are inherent in him. Without an Educator there is no civilization.
Education is of three kinds:
1. Physical - that which tells us about our body and health
2. Material - includes the arts and sciences that we learn about in school and academies
3. Spiritual - this is the most important of the three since it is the one that helps us develop our true nature as human beings by acquiring divine perfections.
In order to progress we need an Educator that can bring about these three kinds of education. This task is far too great for any human being thus, we need a perfect Educator, sent by God. These Educators or Manifestations of God are the ones to bring to us those Teachings that help us organize our matters pertaining our body, technology and sciences and arts, as well as the most important: how to enhance our spiritual nature. Now, this last one is in accordance to our capacity of understanding at the time of the Manifestation's appearance in the world. This means that no Messenger has come and denied the truth of the previous one but instead has given further instructions.
This is why we need an Educator. If a person was to say we do not need the guidance of the Manifestations of God would be as if a child was to say that he does not need to go to school because by himself would attain perfection using his reason and intelligence.
Here is a beautiful Quotation from the Baha'i Writings:
"God's purpose in sending His Prophets unto men is twofold. The first is to liberate the children of men from the darkness of ignorance, and guide them to the light of true understanding. The second is to ensure the peace and tranquility of mankind, and provide all the means by which they can be established."