At this point, at least in NY, all electronic data is demanded in discovery, this is just additional information that can be used in a case. As to why, it could help support certain allegations being made.How so? Not trying to be a smart aleck. Legit wondering.
That might not be the answer you are looking for but its hard to give specifics without an actual fact pattern. I will say, I've used text messages, even encrypted ones, in the child custody part of a divorce.
I don't know how this app is going to work, but if there is something that confirms someones mood and is accompanied by messages it makes it harder to say, "Oh I was joking when I sent that message, It was a miss communication."
I've swayed cases after a court mediator, or the attorney appointed to represent the children, listened to voice mails as opposed to just reading the text. Hearing the anger and or vindictiveness in someones voice can be very compelling.