When you write a murder mystery, you must keep your readers guessing. Any “whodunnit” story needs red herrings to place doubt in the reader’s mind regarding who the culprit might be. No matter how many novels your readers have experienced, they shouldn’t be able to easily predict how it will end. Readers thrive on suspense and wondering what will happen next.
These red herrings may necessitate a few characters who are technically extraneous. These, of course, are the exception to the rule to not include people with no function in your story. The fact of the matter is that they DO serve a function, and that is to keep the reader guessing.