"Need" is kind of a bad word in this case.
Will your snake survive without a temperature gradient? Usually. Will your snake be as happy without one? No.
Snakes thermoregulate, which means they use the heat in their environment to change their body temperature. Body temperature is very important, especially for digestion. Without the correct body temperature, a snake may regurge it's meal. If you don't know already, regurgitation can be very dangerous.
Short version: No, you don't *need* a temperature gradient. But any responsible pet owner will provide one for the health and comfort of their snake.