I think sites like Gransnet and Mumsnet are great for people to air their views. Yes, some are selfish and self-absorbed, but they do get an opinion from a wide range of people. This might make them reasses their thoughts,words and actions, which can only be a good thing.
I don’t think it is helpful to dismiss these people just because others have far worse burden to bear. It is hard to feel that you have been treated badly and have no-one in your own life to discuss it with. The internet opens a world or sympathetic and not so sympathetic listeners.
Nobody is able to post the whole history of what brought them to the site. The posts would be massive, therefore, we only get a quick snapshot of the problem. Hardest of all is when the other person involved puts on a very different face to those around them. To air a problem in real life can often mean that you are not believed. Abusers of all sorts are very good at turning an innocent face the world.
I do hope, @Izabella, that your experiences of the World have not desensitised you to “trivial” concerns of others. Compassion goes a long way in my opinion.