A Matter of Time, Time Played That Is
When people shop for games, they want to know if what they're going to buy is worth the price they're paying. This is just like any product you'd buy, but it's arguably much harder with video games.
Video games are a subjective thing at their heart. How much enjoyment a person has with a game can be drastically different between individuals. Different game genres, art styles, levels of quality, and even developers can make or break a game for some, while others could care less.
So, people do their best to work with the tools available and try to turn a decision that's subjective into as objective a decision as possible. They read reviews written by an author they don't know with scores that are heavily skewed toward the high end of the scale. They watch video footage of gameplay and look at screenshots. Maybe they try a demo. And if they're lucky, they talk to their friends who own the game.
But why don't people share and look at time played? The amount of time someone spends playing a game can be an indicator of if they got their money's worth or not. If someone buys a game and only plays it for an hours or not at all, perhaps it's not worth picking up. At the other end of the spectrum is when a game gets played hours and hours, meaning, for some reason, the player keeps coming back for more. None of this tells you why a game might be good or bad, and you still run into the issue of whether or not you like the same types of games as the person the stats are from. All this considered, it can be used, along with other factors, to help in the selection of games.
Helpfully, Steam keeps track of how long you've played every game in your collection. Below, you'll find a compiled list of every steam game I own and how long I've played each of them. Take it with a grain of salt, especially those games with 0 play time, which could mean I just never found the time to play it.