Cast iron tubs are also supposed to retain the heat of the bath water better.

As mentioned above, cast iron will keep the water nice and hot for way longer. I grew up with a cast iron tub and loved it. Once I moved out of my parents' house all I had was acrylic for the longest time. With my toddler I couldn't fill the tub too much for safety reasons and so the water would turn cold in jus minutes. I have a cast iron claw foot again, finally, and my daughter can have extended play time in the tub and I can take a long hot bath.