This should improve Replay Buffer and Low Latency Networking code, which may result in drastically better streaming with less dropped frames. I don't know why OBS Studio does it this way instead of just calculating the true kbit/sec bitrate by itself, but I guess this is just another case of "it works so it stayed in".
Some of the options shown to the user were never actually being applied, which resulted in encoding that looked worse than it could have. With this these options are now applied and users may experience a sudden increase or decrease in quality if they had these options changed.
In addition to that, some options now use a triple state field, which allows them to also have a default/middle state instead of being On or Off.
Furthermore, log output is now read from the actual AVCodecContext, which contains the actual values used in the encoding, instead of relying on the OBS settings to be the actual values.
Michael Fabian 'Xaymar' Dirks