What does putting a plastic bag over your hair while dying it do?

I've heard and seen people who dye their hair at home, tightly put a plastic bag around their head. I always dye my hair at home and have never thought about doing that. What is it for? I heard it ''makes the color better'', but does it make it darker? Lighter? Enhances the shade? I'm dying my hair a dark brown-purpleish, so if I put a plastic bag over my hair, what will this do? Thanks in advance for the answers. (: