I know that HWM can influence db backup/restore, I don't know where and when in db HWM can possibly act on?

HWM can be simply explained as a tag shows that all blocks after this position are empty, the blocks before this position maybe empty or not. this is also useful for loading and other operations (maybe not visiable for users, but it really acts as an important role, like add or remove containers, add or remove partitions, reorg....)