I used to live in Prague for a while. The button we see here was found in our offices, located inside the men's bathroom right next to the entrance. In a situation where you are in a new country, don't understand all the social rules and don't speak the local language, would you press the button to find out what it does? I didn't. And thus I would pee in relative darkness for the first month there before I got around to taking a picture of the button and asking a colleague about it.
As you might guess from the bright red color and the exclamation mark, it is asking for people to remember to turn off the light as they leave the bathroom. (Lit. "On exiting turn off.") I assume the meaning to save energy (and the lamps). In a sense, the signage of the button performed its purpose very well. Electricity was saved as I used none of it.
I find it semiotically interesting that they would use such strong signals like the bright red color and an absolutely massive exclamation mark here. In my mental model of criticality ratings of buttons, those would be reserved more to things like "FIRE ALARM!" or "HELP!". Though in retrospect this fits into some experiences I had with the Czech vs Finnish culture. Some day I'll try to maybe verbalize what exactly those were.
EDIT: I finally got an epiphany on a walk. My confusion here is simply due to my egocentric viewpoint! The sign is only intended for people who do speak fluent Czech. Any such person will simply note the button (quite easily with the high-contrast red and giant exclamation mark) and read the text. Especially since the bathroom had no other buttons (e.g. urgent alarms) that might be dangerously confused with this one, the aggressive coloring serves its purpose perfectly in the context where we may assume everyone seeing can immediately grasp the text. I note that this matches my other experiences in Prague. When registering as a foreigner living in the city for an extended period of time, I found out that the foreigners' police in Prague does not speak English. It was a bit of an adventure with my level of conversational Czech.