This is a well known convention in Unified Modeling Language (UML®). Amongst other things:
- It is robust against change of multiplicity, the Propery name does not also have to be changed if the multiplicity is changed from singular to plural or vice versa.
- If the multiplicity range is [0..*] or [1..*] there might only be one item anyway.
- In English, it helps avoid issues with properties that have plurals ending in 'ies'.