Why implement a PMO?

PMO (Project Management Office) or may Program Management office ... depending on what you mean by PMO  is not for everyone ...  not necessarily anyway.

Before deciding to go for a PMO, you need to weigh and balance the overheads associated with establishing the PMO versus the expected benefits that may be achieved from it, 

1. One of the key factors in going for a PMO, is the extent (number, density, complexity, diversity, interdependencies, ... ) between the projects that you may need to execute at any one single time.
2. Another factor in going for the PMO is your need/desire to standardize the approach, methodologies, techniques and toold used to manage all these concurrent/parallel projects going at the same time
3. To allow/force project managers to speak the same language with each others and with the central project management authority which is the PMO .  This will help in better coordination, better interdependency resolution, and better utilization of available resources

Hope this is a good starter to open your appetite for the PMO in case you really need one !

I agree with the answer of Mohammad Thiab. I will add my two cents..

PMO body is needed for organizations where a collection of projects are executed. Primary responsibility of PMO is to guide the project managers to follow project management processes, standards and policies identified that are suitable to the nature of projects company undertakes. PMO also evaluates and provide necessary tools for project managers..Change management process, as well as project closure (documentation, lessons learnt, etc) are also part of PMO responsibilities.