if it rains, put up tent. Otherwise bivy usually.
If my insulation is marginal for the temperature I’ll put up tent. It provides about 10F of warmth.
If there’s a lot of dew/frost I’ll sometimes put up tent. But then it gets covered with dew/frost so I’ll have to dry something off regardless.
I’ve measured the temperature of the ground underneath my mat, in the morning when I get up. The ground warms up to about 50F. If camping on snow it’s 32F. I have to add a closed cell foam pad when camping on snow to stay warm.