Why do saunas have two or three different levels?

KLAFS FAQ for well-being

Incorporating two or three levels in the sauna enables sauna goers to choose the temperature they prefer. The temperature on level two is generally around 60°C, whilst on the top level it is around 70°C. If you sit in an upright position on the top level, the temperature at head level is generally around 90°C. Hence you can enjoy different sauna temperatures simply by choosing to sit or lie on the second or third bench.