As GPGPU becomes more and more popular, it is becomes used in applications, which are very far from any graphics or 3D rendering. However, although applications themselves do not need 3D rendering, they need a GL context for GPGPU computations (to send commands and data and get results from the GPU). These may even be some command-line applications, which do not have a window to create context at at all.
So why not allow creating and using a context which is not associated with a particular window? Such a context will be, surely, usable only with off-screen rendering (so it requires FBO to become a standard feature). By the way, DirectX (as I know) also lacks this feature.