The plan for Phineas and Ferb The Movie: Candace Against the Universe! is that anybody can watch it and enjoy it, even those who have never watched Phineas and Ferb before.
But hopefully, after watching the film, those new fans will want to go and watch the series.
Disney+ has a couple of clear goals when it comes to the original content on the service.
Of course, the fact that a given movie or series can’t be seen anywhere else is important, but Disney obviously also understands that in between those original movies, people need to have other things they want to watch.