Why we need buffered rendering ?

[Henrik]

The PSP can render to any location in its VRAM and use as either the scanout buffer (what you see on the screen) or textures. Many games use this to implement special effects, or simply to implement 30fps (you need to show the same buffer twice). We simulate this by allocating an OpenGL FBO for every PSP framebuffer location.

PPSSPP doesn't have complete support for what the PSP can do with these buffers yet, but it's important for accuracy and will only become more so. Turning buffered rendering off is really just a speed hack that happens to work on simple games.