The endoplasmic reticulum is an organelle inside the eukaryotic cell with many convolutions and folds.
The endoplasmic reticulum is a complex network of tube-like vesicles and sacks that appears extremely folded under a microscope. The endoplasmic reticulum consists of a phospholipid membrane holding a fluid-filled space named the lumen or cisternal space.
The endoplasmic reticulum is either rough or smooth.
The advantage to rough endoplasmic reticulum being highly folded is to provide greater surface area for ribosomes to create lots of proteins.
The smooth endoplasmic reticulum uses its many folds to its advantage by providing more surface area for enzymatic action and by also providing ample space to store those enzymes.
The mitochondria also have many convolutions to increase production of ATP.