I think a very important point made above is the value of the one-way aspect of the counselling relationship. It's your time, your space, you don't owe the counsellor a reciprocal relationship.
They are a stranger, so have no personal investment in your decisions or actions, only a professional one. They (if well trained) are non-judgemental and able to explore issues with you from a totally neutral point. It's the counselling relationship that can/should be therapeutic, and this is very different from the loving, intimate support of a friend who knows you well.
I think ideally the two would go hand in hand. Someone looking to vent or offload after a specific incident may well just need an attentive friend to be there and listen. But if you need something deeper, or to pursue a particular course of action, a counsellor would be better placed to help you through.
The following is an extract from a counselling book based upon Wind in the Willows. Toad is having counselling from Heron.
"Heron looked intensely at Toad and realised that, at that moment, Toad's voice and appearance exactly complemented his words. For he looked and sounded, and clearly felt, like a very sad child. This sadness affected the Heron deeply. He sat quietly and tried to share Toad's memories and to experience his sadness, in as far as one person can ever feel what another is feeling. This is called empathy. Toad felt this unspoken support and understanding and it strengthened him to his very soul."