I agree with you for 99% of cases but there can be unexpected domains in the mail, or more interestingly, having your data sold. If hyperrin-nexflux2@port69 starts getting random spams for example, then you have a good idea who fucked up

Because senders don't always use the same domain, and this way lets you block the entire address. If a service leaks your email address, you can just block that email address.