Courtesy to those answering the question
While using/import statements aren't 100% necessary for a good question, I consider them a courtesy to people prepared to look at my question and help me, and I always include them as part of an SSCCE in my questions. It takes me zero extra seconds to copy and paste them into my question, but everyone who looks at my question must take the time to write them in or select them to be put in with an IDE (managing conflicts as they go).
Clarification of ambiguous functions/classes
Additionally, a using or import statement can clarify which of several identically named functions/classes you really mean, avoiding confusion when people have to guess the using or import. I have myself got 90% of the way to telling someone the methods they were using didn't exist, because I guessed the wrong import.
Questions are for future viewers too
Of course, all the people who are capable of answering the question will know which using or import statements are obvious, but a question isn't just between the OP and the answers. It is also supposed to be a resource for people with the same problem, and we cannot guess at their knowledge. As such, removing imports is to increase the amount of 'assumed knowledge' that the question has. Clearly some assumed knowledge is inevitable, but it's never a good thing to increase it.
Truly Unnecessary Code
Of course truly unnecessary code can go, but if it's necessary for people to be able to
copy, paste, run, see problem then it’s not unnecessary, even if "everybody" knows it should go in.
It's worth noting that as a <2k editor even code edits that do just remove truly unnecessary code are likely to be rejected because the reviewers don't have the knowledge and time to check that; that’s a separate issue however.