How?

I often notice that I'm having a slow, regular, smooth breathing, as opposed to a chaotic one (I also used to do micro apneas while working, and I don't notice it anymore). But maybe the bigger effect is that having this in the background helps me be more aware of my body and feelings while I work. I much more often zoom-out of what I'm working on to check my breathing, and also pay more attention to the rest of the body (very often I my case, I notice tension in my shoulders)

If I remember right having a normal heartbeat that's unregular is more healthy then having one with little heart rate variability. What's the case for developing regular breathing?

Breathing with regularity increases your HRV, by making your heart rate follow a sine of high amplitude, instead of being random, something like this. It's quite visible in this video but it's also pretty easy to measure yourself

Also do you have a reason for a duration of inhale and exhale that are the default of the app?

Not really, but I've seen 5 or 6 seconds being recommended by several "coherence professionals" (as with this horrible clock)