Question

Why do insectivorous plantsfeed on insects,if they are autotrophs?

Answer 1

Insectivorous plants are capable of making their own food by photosynthesis but they capture insects for nitrogen.
Insectivorous plants live in nutrient poor environment like in swamp or in wet soil. Nutrients from such soils are washed away regularly. So these plants get their essential nutrients like nitrogen by capturing and digesting insects.