tl;dr the core protocol only lets you inform the compositor that the surface object is "ready" - which turns out to work effectively only for software drawing, or waiting possibly entire frame for your image to show up on screen.
That's if you're careful about calling "commit" on the surface only after whatever GPU draw calls you executed on it finished and the surface has been fully rendered to memory.
Turns out it somewhat works even if you're not properly careful because a lot of drivers (especially Mesa) had unspoken, implicit serialization happening in the drawing pipeline.
One issue is when you have more than one GPU (common on higher-end laptops). Even when you're rendering application surfaces on one GPU, they will often have to be synced to another GPU's memory to render them, because it's common to have some video outputs on iGPU and some on dGPU (example from my 2023 laptop: internal screen is behind mux that allows either GPU to exclusively write, but USB4 DP is iGPU exclusive, and separate USB-C DP is dGPU exclusive).
But with increased use of asynchronous rendering APIs (OpenGL AZDO patterns, Vulkan) and with variable refresh rate screens becoming more common (even if you don't have physical one, RDP/waypipe/etc are possible users for that) you can find out a situation where compositor will start drawing a window using a surface that is not yet fully rendered, bringing back tearing. It also means it's harder to properly utilize VRR in cases where applications render at different speeds (which is fine - not everything has to render at highest possible FPS), possibly locking you down to slowest rendering element.
So, the new extension, available if you ride latest patches on linux (the final part missing is new nvidia beta driver release, but not because nvidia lagged on the issue - nvidia apparently spearheaded it because their driver internals had no implicit synchronization), compositors are able to synchronize their rendering at the GPU level, getting fence objects for DRM buffer based surfaces. Meaning compositor can attempt to prevent drawing an unfinished surface, nor will application have to add extra waits in its display code.
