Project Management: the management of a single project
Program Management: the management of all projects for a single product or application. Projects may be concurrent or consecutive.
Portfolio Management: The management of all Assets which includes but is not limited to projects, programs, applications, hardware and resources. The intent is to align IT with corporate strategy and internal customers so that IT can deliver and maximize return.
Only when you have a strong Portfolio can you then accurately do proper IT strategy and long term planning.
Especially is your organization is large with many different applications/systems. When you get up to the 100's, it's hard to remember details of every application that's out there. An example of this is when looking at long term planning you need to take all this into account to know when you need to upgrade the application to a supported development tool. Or if there is a proposal for a new application, it's good to know what you already have where maybe you can do an enhancement to an existing application to meet your need instead of creating a brand new application.
Those are just a couple of specific examples where Portoflio Management can come in handy.