I think that, in Canada, we are lucky to have services to the public that are provided politely and with a care for clients. That said, a person who requires service at a post office should be assured that they can be served there in the language or languages corresponding to that office's linguistic status.
If I wanted to use English at a post office, I could go to an office that is designated as French, but I couldn't be assured that I would be served there in English. However, if I went to an office with bilingual status, I would have the right, as a client, to be served there in English. It's that clarity that the linguistic designation is trying to provide.