Question

why does pitcher plant eats insects while it can make food by the process of photosynthesis?

Answer 1

The reason for digesting animals is not for energy but for minerals that the animals contain.... 

Whatever the mechanism of digestion, the prey items are converted into a solution of amino acids, peptides, phosphates, ammonium and urea, from which the plant obtains its mineral nutrition (particularly nitrogen and phosphorus). Like all carnivorous plants, they occur in locations where the soil is too poor in minerals and/or too acidic for most plants to be able to grow