* To appear larger: By fluffing up their fur, cats make themselves appear larger and more intimidating to potential threats. This is a defense mechanism meant to discourage attacks.
* To insulate: Raising fur creates a layer of air between the skin and the environment, which provides insulation. This can be helpful in both cold and hot temperatures, but especially in stressful situations where adrenaline is pumping and the body is generating heat.
* To signal alarm: Raised fur is a visible signal to other cats (and sometimes humans) that the cat is feeling threatened or scared. This can help to prevent unnecessary confrontations.
Think of it like a "hair-raising" experience! It's their way of showing they're ready to defend themselves if necessary.