Whether planned or unplanned, knowing how much time you have left in the day can mean the difference to making it to dinner on time or thinking about where to shelter or build a bivy for the night.
Consider using this cool little trick to figure out how much time you have before night falls. Remember that this is just an estimate, so if you want to be conservative, make sure to give yourself a bit more time than you measure if you want to get home before dark. This works best in the moderate latitude ranges (i.e. equatorial to 45 degrees), but if used at higher latitudes you tend to get more minutes than you estimate.
Extend your arm fully and count the number of finger widths between the sun and the horizon. Each finger is about 15 minutes, which means each hand width should be about an hour.