This combination is crucial for compression wear. Polyester provides durability, moisture-wicking properties, and color retention.  
Lycra (also known as spandex) adds exceptional stretch and recovery, allowing the tights to move with the body while maintaining their shape.  
This blend results in a fabric that is:
Durable: Able to withstand repeated use and washing.
Flexible: Offering a full range of motion.
Breathable: Helping to regulate body temperature.
Moisture-wicking: Drawing sweat away from the skin.