Here's why:

* Synthesis and Packaging: The ER is a network of membranes that synthesizes proteins and lipids. These molecules are then packaged into transport vesicles.

* Transport Network: The ER acts like a highway system, transporting these vesicles to other parts of the cell, including the Golgi apparatus, lysosomes, and the cell membrane.

* Specific Delivery: The ER helps ensure that the molecules reach their correct destinations.

In addition to the ER, other components play a role in cellular delivery:

* Golgi Apparatus: This organelle further processes and modifies molecules from the ER, sorting them into different types of vesicles for targeted delivery.

* Vesicles: These small membrane-bound sacs act as "delivery trucks," carrying molecules to different locations within the cell.

So, while the ER is the main delivery system, it works in collaboration with other cellular components to ensure efficient transport and delivery of molecules.