Here's why:

* Ribosomes: These are the cellular structures responsible for protein synthesis.

* Rough Endoplasmic Reticulum (RER): This is a network of interconnected membranes within eukaryotic cells. It is called "rough" because it has ribosomes attached to its surface.

The ribosomes on the RER synthesize proteins that are destined for:

* Secretion: These proteins are packaged into vesicles and transported out of the cell.

* Insertion into cellular membranes: These proteins become part of the cell's membranes.

* Use within other organelles: These proteins are targeted to specific organelles within the cell.

Key takeaway: The RER is a crucial site for protein synthesis and the initial processing of proteins destined for various cellular locations.