They both perform two different jobs, or at least perform a similar job in different ways. Chemical polishes can be used to clean stains, oxidation, etc. Abrasives are designed to remove marring, etc by abrading the paint. I guess they could be used interchangeably, but again, it comes down to using the right tool for the job.
An example would be to use a chemical cleaner to remove water spotting instead of an abrasive polish.
Some polishes may (could) be a mixture of both chemical cleaners and abrasives.