AFAIU this doesn't do a whole lot in practice: OpenGL does not support partial repaints - or maybe it does with obscure extensions. What it may do is avoid updating the screen at all when a window updates only a part that is hidden behind something else.
A quite valuable and IME underused Wayland feature is clients sending an opaque region and the compositor using it to draw windows ("surfaces") without alpha blending where it isn't necessary. Alpha blending uses significantly more fill rate than plain overdrawing. Opaque regions can also be used to determine that a whole window or a region of it doesn't need to be drawn at all. I had implemented that as an experimental optimization in a customer project once. In the end it didn't work out because the 2D acceleration in Vivante GC2000 is broken garbage - they even pulled the documentation of it, but still advertise the feature in marketing material.
It isn't an obscure feature. You can implement partial redraws in OpenGL in the obvious manner, by selecting a single-buffered visual.
This of course has all the expected downsides, like tearing, and modern GPUs are fast enough that there's rarely any point--but it's certainly possible.
Note that Kristian Høgsberg, for years the main developer of Wayland, is among the authors. So I guess it is actually not so obscure in the Wayland context.
It tells you how many frames old the current back buffer is, so you can repaint as necessary to take it from the state n frames ago to the state now.
One could render content into a FBO and simply redraw the FBO texture to screen when necessary. If only a partial update is needed, the scissor box can be set. No extensions are used in this case.
A quite valuable and IME underused Wayland feature is clients sending an opaque region and the compositor using it to draw windows ("surfaces") without alpha blending where it isn't necessary. Alpha blending uses significantly more fill rate than plain overdrawing. Opaque regions can also be used to determine that a whole window or a region of it doesn't need to be drawn at all. I had implemented that as an experimental optimization in a customer project once. In the end it didn't work out because the 2D acceleration in Vivante GC2000 is broken garbage - they even pulled the documentation of it, but still advertise the feature in marketing material.