Insect residue is more acidic than paint and bonds to it if left. On a warm bonnet or bumper, an insect can start to etch within a short time after a drive. Regular shampoo won't shift that. You need a product that actively breaks down the proteins in insect remains.

A good insect remover is sprayed on, left to dwell for a moment, then rinsed off without scrubbing. This removes the residue contactlessly or with minimal friction, without dragging hard particles across the paint.