When would you need facilitation?
If you need a neutral party
...to develop trust from all parties, to make sure that all voices are heard and all parties feel free to contribute.
When there are a number of strong opinions and the results are critical
... to make sure that contribution is balanced and that results are not based on "he who shouts the loudest"
When ownership is required
If you have a team or group of people who all need to buy in to what is happening consider a facilitator who will guide the discussions for equal understanding, clarification and breaking down barriers. They will let everyone have a voice.
When the experience of participants is low and the process for getting to the result is not clearly understood
Consider using a facilitator who will be experienced in the management of the process, make sure that the process flows. They will walk the participants through thinking in an objective way in order to achieve the required result.
The session/ meeting / workshop owner has a vested interest in the outcome
..to make sure that your position is understood by all and is considered alongside other possible positions.
...to validate the owners position and provide support for what the owner is trying to achieve. Gaining joint understanding and support.
...to raise other valid positions that the owner was not aware of which may potentially be a better alternative. Working as a process of due diligence.