Quite often talking things over with a friend can be helpful, and counsellors may encourage clients to use their family and friends for support. But there are some disadvantages of relying on family and friends as your only support or confidant.
Family and friends may have a conflict of loyalty, and they may also find it difficult to keep things that you tell them confidential. They may be upset by the things that you tell them, and also may become upset if you do not accept their advice and guidance.
A trained counsellor will have a formal support and work structure which helps them to deal with difficult and upsetting situations, and are bound by a professional code of conduct relating to confidentiality. With a friend you may not want to tell them something for fear of overburdening them if they have their own problems to work through, but with a counsellor you know that this will not be an issue.