Rough endoplasmic reticulum (rER) consists of a network of rounded or flattened membranous sacs (cisternae) with polyribosomes covering much of the outer surface of the cisternae  Polypeptides synthesized on these ribosomes are deposited inside the sacs for eventual delivery outside the plasma membrane.

Since the RNA associated with rER is basophilic, cytoplasm rich in rER shows basophilia with routine LM stains, such as H&E. This is prominent in neurons, where the basophilic patches in the cytoplasm are called Nissl or chromatophilic substance. Basophilic rER is also prominent in secretory cells, where it is sometimes called ergastoplasm. An example of this occurs near the nucleus of cells in the pancreas ).

For what reason are polyribosomes attached to the endoplasmic reticulum in RER?

Smooth endoplasmic reticulum.