Hockey is unlike any other sport. While many sports are divided into two halves or four quarters, hockey is played in three periods. This unique structure of hockey is not random. It exists for important reasons.  

Originally, when the first game was played in 1875 it only had two 30-minute-long periods. By the end of each period the ice would be full of ruts and snow. Officials had to consistently shovel the ice with shovels during the break. This took a long time between periods, as well as leaving the ice slushier, rather than a smooth playing surface. Causing games to slow down to a near walking pace, passes and shots slowed, limiting the excitement of games. 

Something needed to change the games were becoming dull and unengaging. In 1910, a new rule was drawn up moving to three 20-minute periods instead. Instantly, improving the game players were more rested and the game became much quicker. While also allowing more time for ice to be cleared. Shot and passes improved, increasing the excitement of games, also lowering the risk of injuries in games.  

Still, officials had to use shovels between each period, but the shorter periods improved ice conditions. This changed in 1949 with the invention of the Zamboni. Invented by Frank J. Zamboni revolutionized hockey by creating a machine to create a consistently safe and smooth ice surface.  

As the ice surface becomes worn throughout play, having three longer periods with extended intermissions allows the ice to be resurfaced, maintaining safety and quality conditions. While also helping keep games quick, exciting, and engaging.  

Overall, the rule change allowed the game to be played at a quicker speed throughout the whole game, reducing injuries, and allowing for more engagement.  

https://records.nhl.com/history/historical-rule-changes

https://www.espn.com/magazine/jump_030121_answerguy.html?utm

https://www.nbcphiladelphia.com/news/sports/why-hockey-games-are-played-in-three-periods/3428475/