If you spend time outdoors, it's helpful to know how much time you have left before the sun sets. So you don't end up in the cold, without a shelter, and without a fire when darkness hits. Sure, you can know this based on your watch or other device, but if you don't have those instruments with you,the only thing you will have to rely on is the sun.
So, learn how to read it!!!! This isn't going to work just as it is written here for everyone: you need to understand the area you are in, and properly adapt it to fit that.
Put your hand under the sun in your view and then put your other hand right under that. See how many total fingers it takes to reach the horizon. Each finger will represent a certain amount of time until sunset, depending on where you live.
You should match this to your own location and season (and how fat your fingers are!): Measure the number of finger widths between the sun and the horizon and note the time. Then note the time at sunset. Each finger width will be between 13 and 60 minutes of light. Where most Americans live, it will be around 15-18 minutes per finger. So, you can do it!
Like I said, this isn't my first choice for how to check the time, but if you're in a situation without the necessary equipment to check it, you'll have no other choice, and it's good to know.