Story of Daru Brahma

Sri Purusottama Jagannath is regarded as 'Daru Brahma' (world substance symbolized in wood) both in the Puranas and by the people. The placement of Brahman within the wooden body of God is Nabakalebara. So Nabakalebara is only the change of the covering Daru Bramha, the worn-out body; but the Parambrahman remaining intact. On this aspect Bhagabat Gita puts forth –  

 ‘Basamsi jirnani Yatha Bihahya; nabani grunhati naroaprani; 
Tatha sharirani bihaya jeernanyanyani samyati nabani dehi’

As a man shedding worn-out garments, takes other new one, Brahman or Atman (the embodied soul) also cast off their worn-out bodies, enters into others which are new.  

Our Jagannath cult, follows this principle where the God says that though He is beyond the cycle of life and death, He is one amongst us, His devotees. So He too would die and take a new life for us.

Why are the Pratimas (idols) are made of Daru Bramha, specially Neem Daru Bramha

Barahamihira in "Bruhat Samhita" has mentioned that the worship of the pratimas made of wood or earth or soil bestows longevity, victory, beauty and valour. Idols made of Daru Brahma are considered to be the best as per Shashtriya provisions. Even though the literary meaning of Daru Brahma is a piece of wood, its colloquial meaning is Neem Daru Brahma . Neem Daru Brahma is Dev-Brikhya and worthy of worship like the Peepal tree. Neem tree after ten generations becomes equivalent to Peepal tree and becomes long lasting.

Daru Brahma of the Chaturdha Murti

The Neem tree sought for the purpose construction of Chaturdha-murti must contain some special tributes. By different features of the Daru Brahmas, they are distinguished from each other and so also from the other common Neem trees.