When you are about to start writing a fiction book, you have to decide what you are going to focus on in that book. There are some benefits to writing about what you know - keeping the characters similar to yourself or people you know in real life and keeping their jobs and hobbies related to what you are interested in - but there are also reasons it is better to stretch yourself and write about things you don't know a lot about.
Writing About What You Know Saves You Time:
When you write about what you know, the words flow freely onto the page. You do not have to slow down to research the things that you are writing about. If you enjoy writing quickly and moving through your book without pauses to access outside information, you will enjoy writing about what you know.
Writing About What You Know Keeps Things Comfortable for You:
It can be difficult to research something and then try to incorporate that into your story right away. When you write about what you know, you will be able to talk freely about a hobby or job without worrying too much about how you are wording everything.
Writing About What You Know Makes Your Writing Seem More Natural:
When you know about a hobby, you can mention the tools used for that hobby in your writing in the same way that you would if you were talking about things in person. Your writing will sound natural when you are writing about things that you know a lot about.
Writing About What You Don't Know Can Help You Explain Things Better:
There are times when you know too much about something and you expect your readers to know a lot about that, too. You might use slang words or move too quickly when explaining something in your writing if you know a lot about that. When you don't know a lot about what you are writing about, you will explain things well for your readers, as you are learning along with them.
Writing About What You Don't Know Helps You Grow as a Writer and Person:
When you have to spend time researching things in order to write well, you grow your knowledge and change as a person. You can become smarter and more open when you write about things that you don't know a lot about.
Writing About What You Don't Know Gives You More Freedom in Your Writing:
If you limit yourself to just writing about things that you know, you will not be able to tell all of the stories that you want to tell. When you write about things that you don't know a lot about, you have the freedom to give your characters any jobs and hobbies that you want them to have and have them face any health issues or situations that you feel they should face.
There are reasons that it can be good to write about what you know, but there are also reasons that pushing yourself and writing about what you don't know can be beneficial to you.