I was discussing this very issue with my dear wife yesterday. She comes from the towelling Hub of the Universe, the traditional cotton mill bit of north-east Manchester (Tameside).
We wondered whether it is because towelling is not a very robust material and is likely to go out of shape quite readily, whereas a strip of "reinforcing" helps a towel to keep its shape.
It also makes it harder for the young of carbon-based life forms to injure themselves through ingestion.