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Why we reference
All stacks have an on-going concern with providing high-quality answers to users. Accordingly, every Stack I've seen uses the back-it up principle.
Each site tailors this principle to its own environment. Here, one of the chief concerns is that there is a very low entry to barrier. Most people don't believe they can answer machine learning questions on CrossValidated.SE or explain religious concepts on Islam.SE, but nearly everyone has a political opinion and believes they can answer questions here.
The back-it up principle is what allows the community to distinguish between personal opinion and fact. When you back-up an answer you are claiming that it is more than just your opinion.
As a secondary, but related, benefit: backing-up an answer requires a certain amount of honesty about what has influenced the answer. Everyone has their own personal opinions, but answers need to be more substantial than personal opinion. By asking answers to be backed-up we are requiring transparency about the sources an answer is based on.
When we reference
All that being said, there are two situations in which no external references are necessary.
First, no references are needed when an answer can be constructed entirely deductively. This is a rare occurrence. Even in political theory, where such questions are possible, it almost never happens. It's never true for empirical questions, because you can never control for all possible intervening factors.
Second, no external references are needed when the answerer is the expert and they are relying on their own expertise. Certainly the user should mention their expertise in their answer, but it wouldn't be reasonable to require them to provide an external reference demonstrating their expertise. Of course, by making this claim you are inviting others to determine whether your experience is applicable to the question you are answering.