Snowbooties are meant to protect dogs paws when running on snow.
They prevent the accumulation of snow between the pads and therefor cracks. 
The bootie must be tightened reasonably tight so that it does not fall off. If the bootie falls off, it is too loose or too small/wrong size.
When resting for more than 15 minutes, the bootie must be removed to allow the blood to circulate.