Historically the state of OpenGL debugging and profiling tool is quite a mess unfortunately, especially on OSX and Linux. D3D always came with a matching PIX for capture-analysis (capture frames and step through them), and NVIDIA's PerfHUD for live-analysis (analyse the running game with a realtime overlay). For OpenGL there was gDebugger which was commercial, then free, and hasn't been updated in years and probably is stuck to an old GL version. Then there's NVIDIA Nsight which seems to be limited to NVIDIA GPUs (but has a very nice VisualStudio integration). Apple only seems to care about GLES on iOS, not desktop GL. So VOGL could really fill a gap.
Valve need to encourage game devs from a console and windows centric world to develop for the SteamEngine platform. PIX (http://en.wikipedia.org/wiki/PIX_%28Microsoft%29) is one of the tools these developers are used to dealing with. So having a Linux equivalent is pretty valuable.
It can capture all OpenGL drawing commands made by an application and then replay it frame by frame. Valve devs gave a presentation about it including a demo. http://www.youtube.com/watch?v=45O7WTc6k2Y The part about vogl starts at around 33 minutes and the demo at 40 minutes.
opengl [1] is the graphics library that underpins most of the animation and rendering that games ( and your browser ) do.
This app allows you to trace and capture events in code that calls openGL and to follow what's actually happening.