mmm yes, saving disk space is the pont here... my hard drive is not that big and unfortunately my favourite systems are the CD ones (PS1, Sega-CD, PCE-CD, Saturn, Amiga...)
What is special about the CHD format is that instead of compressing all disc data in the same way, it identifies the audio tracks and compresses them with FLAC (lossless), thats why it achieves a smaller size even compared to 7z or RAR. This is a great feat imo!
.pbp is usually used by people who like to bundle multi-disc games into a single pbp file. Other than this pbp compression is not that special (is +- like the old .CDZ format sizewise). By the way, i see there is a function related to multi-discs in BizHawk but i never took the time to learn it, idk what it does.
I personally never used BizHawk to TAS, and i understand that this whole project is TAS oriented so... if the compression of CDs would mess up stuff, then i guess its a bad idea :-/
Anyway thanks for the replies :-)