Because sometimes it doesn't matter, and where it doesn't matter the option of a silent alarm and a switch off is nice.
My example is yogurt. I start at 21:00, yogurt needs 9h at 43°C, so it's ready at 06:00, a bit before I get up. I let it cool down anyway a few hours before I put it in fridge. To have it stay in the slowly cooler getting water is no problem at all.
There are certainly other scenarios, probably the one you thought of when you disagreed - and you are perfectly right under your circumstances and I agree fully with you, that why I think more options are better.
Another 'switch off after time is up' scenario is the preheating of milk before yogurt making (82°C/30min) to break down the proteins in fresh milk.