Why are parameter names necessary in an interface definition? I am allowed to choose new parameter names during implementation

Parameter names are required in an interface declaration for clarity of implementation and for reference. If someone were using your interface, the names of the method parameters are meant to be self documenting so the consumer of the interface understands what to pass to the method (eg when viewing the method description via IntelliSense)

And yes, when you implement the interface you can name the parameters whatever you want.