I think there is no difference -Voicy vs. Saera- in making them do anything useful. Saera does all the processing in one file and Voicy separates these things into different files, one for each app to control. This separation only makes it easier to add/remove controls for a user. The actual piece of python code needed to control something is the same.

From my perspective there aren't many situations where voice control for a phone is really useful (I am not talking about fun things like Chuck Norris news etc.), but at least there are some.
My list goes as follows:
1: navit, because it's better to concentrate on driving and to keep the hands off the phone
2: accept/reject a phone call while driving
3: dial a number while driving
4: control the mp3-player (for example in bed)
5: take a picture

Any suggestions for other useful things?