One of the realities of leadership life is that there can be competing interests or positions in organizational leadership. Having a facilitator ensures that a neutral party is able to guide the discussion, and identify any conflicts or other issues that might be impeding achievement of organizational goals.
They are usually a full day or half day. I do a comprehensive orientation in advance to make sure people are well-prepared to contribute at a high level during the actual retreat.
I’ve facilitated retreats for many different types of groups. Most often it’s for senior management, but I’ve also facilitated them for entire divisions, and entire staffs. I’ve found that the best outcomes occur when participation is just among senior managers, who then can facilitate strategic planning with their own units or divisions.