There are multiple use cases for a pre-save hook in the file handling. 

I may want to have multiple copies of a release i.e. a flac, a low compression and high compression copy for different uses. I want to be able to set my own disambiguation patterns instead of rely on the built-in routine.

A plugin that wanted to handle sidecar files or make backups would also find this function useful. See PICARD-2495 and PICARD-2820 for potential uses. 

The post save hook is too late - the file has already moved, and nothing preserves that information.