Here's why:

* Rough ER: This part of the ER is studded with ribosomes, which synthesize proteins. These proteins can then be transported to other parts of the cell or even secreted outside the cell.

* Smooth ER: This part of the ER is involved in the synthesis of lipids and steroids, as well as detoxification of harmful substances. These products can also be transported throughout the cell.

Other organelles that contribute to cellular transport:

* Golgi apparatus: Processes, packages, and sorts proteins and lipids produced by the ER for secretion or delivery to other organelles.

* Vesicles: Small membrane-bound sacs that transport materials within the cell and to the cell membrane.

* Plasma membrane: The outer boundary of the cell that controls what enters and exits.

* Mitochondria: Generate energy (ATP) for the cell, which is essential for transport processes.

While the ER is the most directly involved, cellular transport is a complex process that requires the coordinated effort of several organelles.