Pool blankets are multi functional, and although predominantly designed to insulate and keep your pool warmer during those shoulder months, many pool owners use them during winter to keep debris out of the pool, and to save on evaporation of chemicals during the months that the pool seems dormant and is not used quite as often. (If at all during mid winter)
Remember if you're using a pool blanket in winter to ensure that you adjust your chlorinator running times, or chlorine output accordingly. Your chlorine efficiency is higher in winter, due to the decreased temperatures and less evaporation. High chlorine will quickly deteriorate your pool blanket.
A pool blanket is also very beneficial to save on chemicals. Due to the cover, there is substantially less chlorine evaporation and sunlight's effects on your chemicals, and hence, reduces the need to add stabilizer (otherwise known as sunscreen)
