This is how games were written back in the day before DirectX was a thing. You'd write directly to the frame buffer and instead of clearing and redrawing, you'd redraw what changed and what was around and under it (because there was no time to refresh the entire view in time in addition to everything else you need to do)
The first is to write to another buffer (possibly in normal RAM, not video RAM), then when the frame is done copy the whole buffer at once, so every pixel gets changed only once.
The second is to write to another buffer that must be in video RAM too, then change the registers of the graphics hardware to use that buffer to generate pixels for the monitor to show.
They had different tradeoffs. Copying the whole buffer when done was expensive, changing an address register was cheap. But the details of the register were possibly hardware-dependent, and there was no real graphics driver framework in place. Also, to just "flip buffers" (as changing the address register was called), rendering to the off-screen buffer meant sending pixels to video RAM, which was (IIRC) slower to access than normal RAM (basically a NUMA architecture), so depending on how often a pixel gets overdrawn, rendering in normal RAM could be faster overall even with the final copy taken into account.
Did this change with 3dfx's Glide (or subsequently Direct3D once Windows got a foothold into the gaming industry)?