Why did John bother emailing me about the assignment if he didn't need our consent?

Notifying the other party that you've assigned your rights under the contract is legally required. John had to give notice.

Notifying the other party that you've assigned your rights under the contract is best practice. John was smart to give notice.

Notifying the other party that you've assigned your rights under the contract is not advisable. John shouldn't have given notice.