Problem
When I am writing an answer, especially a detailed one, it's useful to be able to refer to the question to make sure that I am answering everything and doing it in a logical order. Because of this need to refer back to the question, I have to scroll up and down the entire page which takes a while when there are lots of existing answers. This can also be annoying when a lot of the answers I am scrolling past are ones that are greyed out because they did not answer the question.
The crux of the issue is that Stack Exchange is a question-and-answer site, but your answer and the question you're writing an answer to are placed on opposite ends of the page.
The current page structure looks like this:
Solution 1
One (and my preferred) solution is to have a "Write an answer..." button underneath the question which when clicked on expands to reveal the answer box directly below the question. That way the question and your answer are side-by-side. You could still have another "Write an answer..." button at the very bottom of the page which jumps to the question and does the same thing.
Solution 2
Another solution could be to add buttons (which only appear when you have entered something in the answer box) to quickly jump between the question and your answer: