Let's say you have two relations: Employee (Id --> Name, Department) and Department (Id --> Name). You're asked to join them by equating Employee Department with Department Id, and not conflate the different Name attributes.
In SQL, we can specify the fields we want to join on, but in relational algebra, the join operators are defined based on equally named fields. Parameterizing join operators would make the algebra unnecessarily complex. It's much simpler to define a rename operator.