A frequent issue in my house is that someone will use the bathroom and either forget to turn the extractor fan on, or they turn it on and forget about it till they or someone else next visits the bathroom — leaving it on for hours.
Additionally, I find the noise of the bathroom extractor fans to be particularly bothersome.
Lastly, when guests visit, I don't want them to feel self-conscious about needing to turn the bathroom fan on and having everybody nearby hear it.
This is one of the simplest automations in my home, but it gets used multiple times a day, every single day:
When bathroom lights turn off
Turn on the fan switch
Turn off after 10 minutes
This ensures that the fan is always turned on, but only for 10 minutes, which is ample time to air out any less-than-desirable smells.