Because of the intent to make sure some things are done or not done on certain days, there are various ways of measuring or defining when evening has arrived: sunset or the visibility of 3 stars in the night sky. So, if you want to make sure you don't do something on a certain day, like work on Shabbat, you'd stop doing those things as the sun sets. But if you want to make sure the day has arrived, you'd wait for 3 stars.