I've never used Flo specifically, so I don't know what kind of data analysis it has available, but period data is the #1 most useful health data to have an app crunch for you, and "your period starts tomorrow" is a pretty darn useful notification to get.
We're using Flo specifically, mostly for sharing stuff like "her period starts tomorrow" to the both of us, she doesn't really need a notification for that :)
Most of the women I know well enough to know this about them track and predict the onset of their next period without needing an application. It isn't exactly rocket science.