I was going to share a graph from when I put a DHT20 hygrometer in my bathroom to prove to my family that the humidity was the cause of the mould and they should stop turning the dehumidifier off when its built-in hygrometer said it should be running, but unfortunately, it was long enough ago that Home Assistant decided I no longer need my one-every-ten-seconds readings and now only shows hourly readings, which aren't enough to prove my point here. You'll just have to take my word for it that when I did this test, I was surprised to find that although the humidity at the other end of the room started rising quickly after the shower was turned on, it peaked fifteen or twenty minutes after it was turned off again because diffusion without something like a fan or a draught moving the air around can be really slow.

My bathroom's a weird shape as it's long and thin and has a weirdly high ceiling at one end, so it's not going to have typical airflow, but it is a real bathroom that really exists, and I did have data in the past showing it dried out faster if I manually turned the dehumidifier to maximum (so it would run even if its hygrometer said not to) ten minutes before turning the shower on than if I did it immediately before turning the shower on. Whether I'm going to shower in ten minutes is something I can know but a hygrometer can't. This isn't even really related to whether the dehumifier is smart as mine isn't and I can operate its switch as easily as I could operate a smart switch, and my shower isn't electric, so there isn't a switch I need to operate before using it that could be made to do two jobs