At Special:PageMigration, when the h2 alignment results into many units getting collapsed into a single unit, it becomes a tedious task to separate the chunk and the user has to do a lot of cut-copy-paste work.
However, in most common cases, to align manually, it is just required to bring the collapsed text next to the source unit and the consequent units would get matched automatically.
Having a split feature - which splits the headers from wikitext would automate this process and there won't be any need to cut-copy-paste. All the user has to do is bring the collapsed text next to the concerned unit and click the split icon.
This is more like a user controlled version of https://gerrit.wikimedia.org/r/#/c/136334/.
Collapsed text is just a use case of this. It is also useful when there were no newlines present before/after headers in the translation, where splitting becomes a job of a single button click.