It doesn't seem to do anything that a normal "Dating" app does. It's just a private friends list. You can already like and message people you have a crush on, without this. Who is this for?
"Secret Crush lets you match with people you already know on Facebook and/or Instagram."
Because what people have been clamoring for is to know which of their friends like them more than is mutual, so they can feel weird.
"You can choose to see other people who are using Facebook Dating that fit your preferences within the groups you are part of and the events you have attended or will be attending."
'Oh look, this person at this upcoming event is looking for a date! I'll go hit on them in person without messaging them.'