Most people separate their loads of laundry to keep like colors and fabrics together. This does more than protect your clothing, it can save you energy as well.
Separating heavier cottons from lighter weight laundry saves energy when drying. Lighter weight clothing dries faster, thus using less energy. When combined with heavier cottons, their drying time increases.
When you separate clothing, try to make sure to wait and wash a full load. It costs practically the same amount to wash a small load of laundry as it does to wash a full load. If you do two small loads of laundry, you have just double your energy costs.