By playing tension free you are activating only the muscles you need when you need them, which saves them and keeps them fresh for when the time comes. Making you more efficient and leading to less fatigue

A tense body lacks agility! In order to move quickly and fast you must start from a place of relaxation, because in order for your muscles to re-fire, they first need to relax. 

Try this - Flex your legs as hard as you can then try to make a quick side step… notice that your legs have to first relax in order for you to move. 

When your default is a place of relaxation you eliminate the pause between contracting and relaxing so you can be faster and more agile!  

Tension in the body increases the risk of injuries. Relaxed muscles are more flexible and less prone to strain or tear during intense gameplay. Thus, you can play the game you love for longer!

Your brain has a limited amount of processing or computing power, the more tension you hold in the body and mind the more you are using those limited resources that should be going to processing your environment and making decisions. (Simplified explanation) 

Think about trying to solve a simple math equation like 5+7 while doing a max deadlift vs doing it relaxed in the comfort of your own room

When you're playing tension free you are able to see the ice better, have greater field of vision, process information quicker, think clearer and overall make better decisions on the ice. 

Overall, stress clouds your judgment and dilutes your senses leading to poor decisions and poor in game results.

When you're relaxed, you're less likely to become frustrated or angry, and thus keep your emotions in check during the game.

This allows you to enjoy the game, have better interactions with teammates, opponents, and maintain a positive attitude throughout the game.