With parent pom, you can only import one, with bom, you can import as many as you like. This allows you to start building re-usable bom files which you can then import into new projects without having to copy-paste all that bits and pieces from another project and then maybe getting the version wrong of one dependency.
Also, the version management is done inside the bom file, so if you need for example hibernate, all the compatible versions of components are inside the bom file and you just pull in the bom file without having to worry about importing the correct version of each of the related dependencies like hibernate validator, c3p0, caching, etc.