Sometimes, the website effectively is the business. Sometimes it's just meant to serve as a business card. And all other imaginable scenarios fall somewhere in the middle. This makes your job, as a designer, quite difficult.
Did you ever have to put aside a project to respond to the 15th revision request from a client? Being the professional you are, you'll keep working until your client will be satisfied. Yet, who has so much time?
Knowing the difference between good and bad photography will help to make your design more appealing. Think of this: you are designing a website, some photos were given to you to choose from so you can attach one of them as a header image.
If you draw a comparison between web design and architecture, you will see that the modern minimalist design (aka flat design) and the tabula rasa pioneered by the European modern architects at the onset of the 20th century are similar in more ways than one.