The usual advice is to keep at least the most recent CBS.log file, but really, it's only useful for troubleshooting. When CBS.log gets to about 50 MB, the system is supposed to compress it into a CBS.persist file and start a new CBS.log, but it seems that compression operation can fail if the logs are big (> 2GB), if there isn't enough free space, etc. If SFC reports that if has found corrupt files but can't fix them, you can examine CBS.log to determine what those files were. files are created (at least) when the System File Checker is run (the usual command is sfc /scannow).
