Significance of Nissl bodies
I am going to try answer this question.

The origine of this name came from article writen by Franz Niessl (18660-1919) and published in 1894. HE found this substance after he visualized it by nissl stainning method invented by him. 

This basophilic substance was just only find in dendrites of neurons, not axones. It is composed by RNA polyribosomes.

Basicly i am tring to tell you that Niessle body is just older name from old ages of histology. 

Niessel body is not diffrent with others rought endoplasmatic reticulum(RER). It same pourpose as normal RER.