Cobras have spring-like tissue connecting their vertebrae that can store up a great deal of energy, which they release when they strike. I'm not sure we know why they have hoods, but they do use their hoods to appear bigger and more dangerous (even more so than usual) when they feel threatened.

Cobras are super-fast and have a hood because their parents and grandparents and great-grandparents were super-fast and have a hood. Cobra ancestors had more children and grandchildren and great-grandchildren if they were fast and had a little bit of a hood.

The DNA in our genes and chromosomes makes us that way, and it's like the DNA in our parents' and grandparents' genes and chromosomes. This is the result of evolution.

Cobra's hoods make them look bigger, so they open their hoods when they are scared.

Why Does a Cobra Have a Hood? I took information for you from the following link, which is serios and interesting:
Sciencing
"The cobra's hood is created by many elongated ribs that are capable of extending the looser skin on the neck outwards. The cobra rises up on the forward portion of its body and flattens its neck, spreading out this skin on the ribs to create what appears to be a hood. Cobras do this whenever they are disturbed or feel that they are in danger. By doing so they can make themselves look larger than they really are and potentially unnerve a predator or foe."