So it can notify you when the water has reached the set temperature or the time you set for cooking is up. Which can be handy. However, I found the BT very weak on my Anova and it would lose connection when I went into my home office a mere 25' away, so I stopped using it. There's actually no need for the water to be up to temperature before you put your food in, and food can sit as long as you want; half the point of sous vide is to be able to hold food at temp without overcooking. So you don't really need the timer either.

I have something similar, but wifi. Never even tried to connect to it, because you just use the buttons to set temp & time.

I can imagine, though, that an app might have buttons for 'eggs', 'yogurt', 'steak', etc. Or maybe let you program temperature-time sequences. Or let you check how much time is left from the next room. Conveniences. Definitely no need for them to phone home, though, except maybe for an ad-driven 'recipe of the week' type thing.