The person has to tell you that to make sure that you are informed. If you simply go there to get a form or to ask a simple question, she will answer you in French. However, because you must be informed, she will also have to tell you that, as a person who wants to be served in the other official language, you can also have fully bilingual service. She will also tell you where this service is available to you.
For example, if you come to the office because you want to know whether you are eligible for employment insurance, it might get a bit more complicated. In that case, the person will not have any supervision in French, she will not have the organizational support she will need to give you the best possible service, equal in quality to the service you are entitled to receive, in the language of your choice.