We should not allow links to outside Websites in either the questions or answers. The idea is that WE are the destination site for users - it seems a bit lazy!!
There might be additional related information in the question or an answer.
Also, you need reason to believe the information. There's often a need to provide reference materials (eg, FDA guidelines, if nothing else). We do not want people copy & pasting information and introducing plagiarism issues without correctly citing the source.
Answers also need to be short and to the point. I prefer to direct readers to more detailed answers and explanations elsewhere. As Joe mentioned, it also helps validate the answer in the mind of the reader.