Why do hair salons put tin foil on their hair when they are dying it?

Hair salons will paint the hair with the dye, and then they will wrap it in tin foil, and i was wondering why they do that, what does the tin foil actually accomplish?Why do hair salons put tin foil on their hair when they are dying it?
It keeps it from spreading to the other parts of the hair you don't want dyed. The foil is usually used only for highlights or certain things you want done to one part of your hair.Why do hair salons put tin foil on their hair when they are dying it?
Foils separate colored hair from non colored hair. They may use different colored foils for different colored high-lowlights.