No matter what you know or think you know about a subject, there is always more to learn. One of the most embarrassing situations to have happen is being called out because what you thought was true was incorrect. Always make sure you have the latest information available on your subject. If there are differences of opinion in the area you are writing about, recognize the other side. You will come across more strongly if the reader knows you have taken the time to learn both sides of an issue. It is amazing but true, that once you write something most of your readers are going to believe you. You owe them accuracy.
What if you are writing your autobiography or maybe your family history. Who better to I know the story then you? It’s true, but others may have different perspective. Memories, even yours, can be faulty. Checking your facts can lead to the perfect tale. I was editing a memoir the other day and the author made a reference to an event that took place during World War II. The problem was that it never happened. It was related to a post war event and had to be moved. A little research could have solved that problem. If you’re not sure check it out.
What if you are writing fiction? Surely doing research is not needed is this case. After all you are making things up as you go along. The truth is that the details of fiction need to be as accurate as the details of nonfiction. There is always an element truth in fiction and those details need to be correct. For example if you are writing about a sword wielding maniac and you call his blade a rapier, there is a problem. Even when you invent your own universe, it has to be understood by earthlings. If you are going to have impossible things happening, you need to offer some explanation that will make sense.
I myself love to do research. I look to facts to build my stories and locations on. It gives your reader a starting reference and will make them feel comfortable with the subject matter.