What does the region even do? why not just stay in france mode?

Region affects pricing, what payments methods are availanle or accepted and also what apps are available. Some banks, government service apps and few commercial companies limit apps to certain regions.

Unlike Android there are way more hassle in installing iOS apps under different accounts with different regions. On Android I just have 5 diffetent accounts with different billing countries and it just work.

Also AFAIK iCloud family features dont work cross-region.