Why some plants eat insect?

Firstly plants don't eat insects; they catch and digest them, there is no mechanical "eating" involved, although the "teeth" along the edge of a Venus flytrap leaves often give rise to this notion.

Plants which catch and digest insects (and sometimes small mammals) are called for the main part "insectivorous".

These plants are found in a number of locations around the world and have developed various modifications over thousands of generations in order to evolve to this point.

Mainly these plants grow in Nitrogen poor soils, these often occur in tropical or sub-tropical regions or wetlands. All plants need Nitrogen to grow; this is one of the fundamental building blocks for plant growth.

In order to supplement the lack of organically available Nitrogen these plants have adapted to capture and digest insects (which are high in Nitrogen). These plants are all green, contain chlorophyll and are able to photosynthesis their own food.

Even supplementing their diets with insects the amount of Nitrogen absorbed is not huge, this limits the size of the plants, so the chances are you will never encounter a giant man-eating Venus flytrap while stumbling through the jungle ;)