What gain is achieved by double buffering as opposed a single buffering?

Buffer is like bucket of data you write to at your own pace so that some other process can read from it at its own pace when it is needed. If you use a single buffer, you may end up overwriting what the other process did not read yet, or the other process may "catch up" with you before you finish writing your current set of data. Basically, you still need to carefully synchronize the two processes. With two buffers, you can write to one buffer while the other process is still reading the other buffer.