Here are some random resons for a PMO.  Please feel free to add your favorites.

• To prevent project or business initiative failure
• To increase PM capability
• To increase corporate project capability
• To manage project capacity
• To ensure project consistency
• To provide program/project management capability and standards
• For project resource management and reporting (either company-wide, division or department)
• To ensure the organization keeps commitments...Need enterprise/global view and control of resources, costs, project portfolio, and status
• The PMO provides more acountability - Prevents project from being never-ending and takes ownership for issues and escalation at the enterprise level
• Provides standardization of tools and processes
• Provide better PM leadership
• Provides staffing and reporting for Governance
• Ensures selected projects get started
• To help restructure the organization
• Used to impose change
• Provide training, tools, and standards
• Estimation, PM software solution training, scheduling, methodology, etc.
• Stage gate and other project reviews
• Knowledge management
• Management of relationship between project cost and ROI...works with Governance on benefits realization
• Corporate and cross-project risk and issues management
• Corporate spokesperson for the value of project management 
• Linking project delivery to the corporate strategy and goals

I think you missed one: So the management team can avoid project management!

Seriously, they (correctly) view PM as a discipline that (incorrectly) gets in their way, but is good for all the low level-people on the projects.  In at least some instances, PM serves as a substitute for for supervision. 

The book "Corp Business" describes the business of the Marine Corps and discusses many of these topics in an interesting way.  When you think of this in relation to the Corps, the Marine Corps does projects varying in scope and location, locally to globally, with interaction and suport from other groups.

It is an interesting read and gives a different view to the PMO.

Corps Business, by David H Freedman is availabe on www.amazon.com.

I agree.  As long as you have a charismatic PMO leader and full support from the executive team with a clear charter for the PMO, you have a chance to succeed.  I run the PMO Roundtable SIG for PMI in the Seattle area and one of the conclusions we have come up with is the PMO has to re-invent itself about every 6 months to stay fresh and alive.  There are many things the PMO can provide including:

·         Training/coaching/mentoring

·         Center of Expertise for the methodology and templates

·         Center of Excellence for metrics/reporting/status

·         Focal point for project reviews, gate reviews, Change Control, etc.

·         PM resource pool/resource management

·         Member of the Senior Management Team

As long as the PMO can provide value to the Senior Management Team, they have the possibility of staying in business.  If they stagnate into just providing metrics or the monthly project status report, the value quickly diminishes.  Evolution and re-invention is the key.