At times, people do not have the money nor the capacity to go and see a doctor for a diagnosis. In these cases, it would at least behoove the person afflicted to seek advice from others online.
In another case, some people who suffer from major depression may not realize it at the time, and they may say things on the internet which allude that they do in fact suffer from depression. In these cases, other people could recognize that this person is suffering from major depression, and could aid in counseling the person or advising that the person find help from a friend or a close relative - in so doing, the person giving this advice would have A. Diagnosed the person with depression, and then B., given the person an opinion on how the person should treat their condition.
It is irresponsible in the extreme to take the advice of an Internet forum over a doctor, but I can see in certain situations that it would be beneficial. Perhaps one of the main benefits that I can see is that certain strange symptoms that a person is having might match certain diseases and cause people to seek out medical care.
Given my personal experience with PCOS, I think it can be vitally important for people to take Internet forum content into consideration when learning about their own medical problems. I don't know that they should use it as the only source of information and they should certainly talk about things with their physician, but being more self-aware can be helpful. For me personally I've been told by countless people that Metformin does not regulate periods, but my personal experience and the experience of other PCOS patients, it most certainly does. So, yes, it can be helpful.
For minor medical decisions like what brand of medication of take, internet forum content might be useful, but major decisions should be made after seeking the advice of a professional. The internet can be misleading, and doctors have been training their entire careers. Getting help on the internet may be cheap, but seeing a doctor just might save your life.
People should not use information they see on the Internet to make medical decisions. It is necessary to consult a trained doctor when facing any illness. Information on the Internet might be incorrect, and it could lead people to make decisions that are bad for their help and which could hurt them.
Internet forums are probably the worst possible place to get medical advice. No matter what forum you go to, they'll end up convincing you that your headache is a brain tumor. Or that your zits are a flesh eating disease. Go to a doctor (you have no excuse with Obamacare) and get a real diagnosis.