Final answer:
Ribosomes attach to rough ER for the synthesis and modification of proteins.
Step-by-step explanation:
Ribosomes attach to the rough endoplasmic reticulum (RER) because it is involved in the synthesis and modification of proteins. The ribosomes on the rough ER produce proteins that are destined for the cell membrane or for export from the cell. These proteins are synthesized within the ribosomes and are released inside the channel of the RER, where they undergo modifications such as folding or addition of sugars.