If you are not aware of which cloud formations are conducive to beautiful sunsets, you will waste valuable time and film on an inferior sunset. And, if you have not checked around and found out what makes for good sunset photographs, you will not even be aware that your sunset pics are inferior.
If you do not know that the final minute of a sunset is most critical you might miss the perfect moment. The sun descends at such a rapid rate that you can actually see it move. Thus every second counts.
And, if you are not aware that you shouldn’t just take a direct reading with your meter, you will end up severely under-exposed because the sun is aimed directly at your meter. You must open up at least two to three stops in order to get proper exposure on the clouds and water where the light, that gets to the film, is a lot weaker.
This photo is an example of one option a photographer can choose when shooting a sunset. Here the photographer has chosen to have the water and clouds exposed properly while living the people underexposed and creating silhouettes.