It helps because you will have to draw them over and over and even if you have a general idea, most people can't track every detail.
Sure you can go back an look at previous panels, but that is time consuming, especially since panels rarely show every detail. It can mean going through twenty pages to find the last time you drew shoelaces or the watch face.
It also helps give you a base reference for colors that change in different lights.
That doesn't mean you need to do a fully rendered piece. Personally, I need to get better at doing character and environment references, but I do a quick clothing sketch at the start of every major chunk where clothes change and it's super helpful.