Why do the incert-catching plants need to catch inserts?

Insect catching plant are green. They can do photosynthesis.
The catch insect mainly because they want minerals such as nitrogen and phosphorus, which cannot be produced by photosynthesis but is important in growth.
A lot of other plants simply get these minerals from soil. However, in places where these insect eating plants live, the nutrient is not enough or unavailable in the soil. So these plant attack / catch insects, digest them, and get the minerals out of the dead bodies of the insects.