They exist as a form of self-help for the person who is or may be using the site/app.
In theory they're a good thing. They answer the questions that people want to know and they don't have to directly contact the provider of the website or service. This saves time (and money) for both the asker and askee.
Do they actually help?
If you have them on your site are you checking if they are actually answering the questions people have and/or solving their problems?
Are you checking that the people who read them are getting the answers they want or need?
Are they really 'frequently' asked?
How was the list first populated? Did you just guess at the questions you thought people would ask? or are these the questions that people are asking (during early testing) and you don't have a better way of responding to?
Are you monitoring the questions that people are asking to you? and updating your FAQs appropriately?
If they're not frequently asked, do they need to be in the list? Taking up space and potentially hiding more valuable questions/answers.
Could you do without, at least, some of them?
Is this the best way to respond to genuine questions from customers or prospective customers?
If these questions are important could they answered in a better way than hidden in a list?
If the question is about the benefits or use of the site/app, could the benefit not be made more obvious or the site/app be made easier to use?
If the question is about an error or problem with the site/app, is this being addressed and the FAQ only displayed, temporarily, until this is fixed?