What is the use of lid in a pitcher plant?

This trapping mechanism is for the Trumpet pitcher only.

The lid is basically an advertisement in the pitcher plant. The bottom side of the pitcher plant is covered with nectar and microscopic downward pointing hair. When any insect comes to collect the nectar the ant gets a downward thrust from the microscopic hair and falls into the liquid. Without the lid the pitcher plant will not be able to catch insects.

Referance: David Attenborough [BBC Wildlife]