It’s important because of this:
from this page: ℹ️ Timeline display rules
If you don’t remove the title element from the json file, micro.blog will display the post this way (title plus link, no summary).
As for why my code isn’t working for you, I can’t say. Maybe play with it a bit? Maybe you’re writing longer summaries than I am, and something needs to be tweaked? I don’t really know - I guess just check whatever you’re doing against the timeline display rules above to make sure your summaries aren’t getting caught in one of those conditions unintentionally.