Many people use these to protect their feet, especially where there may be sharp stones, however their real purpose is as a protection from colder water, enabling you to swim longer and earlier , or later, in the open water season.
BlueSeventy have now introduced Thermal hat, sock and gloves that have an insulating layer on the inside - very toast warm. BlueSeventy have also introduced a Thermal Helix Wetsuit
A little known rule is that on an Ironman swim if the water temperature drops below 18.8 degrees [common in the UK and Europe], neoprene swim socks are allowed. here is the precise rule
"Neoprene socks are allowed ONLY when the water temperature is 18.8 degrees Celcius/65 degrees Fahrenheit or colder."