There is another use case, that happened in my last job: migrate from one framework to another. We were using EmberJS, which is a framework that I liked a lot,

There is another use case, that happened in my last job: migrate from one framework to another. We were using EmberJS, which is a framework that I liked a lot, but it was just almost impossible to hire frontend developers to work with Ember. It is such a niche framework that it is basically asking developers to compromise their future career prospects to work on it at this point. We used Single SPA to make an organized and gradual transition from Ember to React. So having a solution to run different frontend frameworks is a way to avoid lock-in on a single framework (which can go out of favor in the future for whatever reason).