are they just… using it till it evaporates?

Yes, but I’d say it goes further. Using it until it evaporates isn’t a side effect of some other cooling process. Evaporating the water is the cooling process.

Signal is gaining the network effect. Session is not. Don’t let perfect be the enemy of good enough.

I think the idea is that for one, the front screen can be smaller and more easily used because the main screen is bigger. On the other hand, the main screen is bigger, allowing it to act as a tablet without having to carry a second device. On top of all that, it still fits in your pocket.