Here's why:

* Ribosomes: The RER is studded with ribosomes, the cellular machinery responsible for protein synthesis.

* Protein Translocation: As a protein is being synthesized by a ribosome, if it is destined for secretion or incorporation into a membrane, it will enter the lumen of the RER through a protein translocator.

* Folding and Modification: Inside the RER, proteins undergo folding and various modifications (e.g., glycosylation) to ensure their proper structure and function.

* Transport Vesicles: Once modified, secretory proteins are packaged into transport vesicles that bud off from the RER and move to other cellular compartments like the Golgi apparatus.

Therefore, the RER serves as the central factory for producing and processing proteins destined for secretion or membrane integration.