Why does each MPEG-2 group of pictures (GOP) begin with an Intra-coded frame (I-frame)?

Each MPEG-2 group of pictures (GOP) begins with an Intra-coded frame (I-frame) primarily to serve as a reference for editing or commercial insertion. I-frames are self-contained images that do not rely on other frames for their data. This characteristic makes them ideal for serving as reference points in video content, allowing for precise editing and easy insertion of advertisements or transitions.

When video is edited or when commercials are added, it is essential to have a clear and intact frame to start from. The presence of I-frames at the beginning of each GOP provides solid anchor points in the video stream, ensuring that any edits are clean and that the integrity of the video is maintained. This is particularly important in broadcast and streaming scenarios where content might need to be dynamically modified.

While aspects like playback speed and buffering are important in video streaming, they are not the primary reason for the I-frame's positioning at the start of each GOP. The emphasis is more on the editing flexibility and the structural integrity provided by the I-frame in the video encoding process.