I can't speak for Canada, but in the US a formal diagnosis makes it easier to get prescriptions or other treatments, such as therapy.
As far as required disclosures, you aren't required to disclose anything to anyone. However, disclosing a diagnosis to an employer often opens up their ability to provide accommodation. The ADA also prevents employers from discriminating based on disabilities, and 'disability' in this context tends to be interpreted very broadly.
An example from real life: I am a manager at a grocery store, and one of the people on my team has some kind of ADD/ADHD/autism spectrum thing going on. It's pretty obvious, I recognize the symptoms. However, he has not disclosed anything to me, so I have to pretend I dont see it and treat him with the same expectations as everyone else.
If he were to come to me with a diagnosis, I can ask him how I can help him, what he needs to be able to do well, what sorts of things tend to distract him, etc.
Once again, this is all from south of the border, though, so my takeaway would be to see if there's any similar legislation in the Great White North.