What's the purpose of customized record settings of I、P、B frames in MPEG-1 file format?

MPEG has adopted 3 ways to compress every picture frame, which are "I frame" (Intraframe), "P frame" (Predicted frame) and "B frame" (Bi- directional frame).

These three frames are explained as follows: I-Frame (Intraframe pictures): I-frames typically are the first frame of each GOP moderately compressed, and serves as the reference points for random access and can be likened to images. P-Frame (Predicted pictures): P-frames are coded with reference to past pictures, predict in a forward direction, and are moderately compressed. B-Frame (Bi-directional pictures): B-frames' predictions are forward, backward and bi-directional, which is relative to the other I-frames and P-frames. Compression is at a high level.