I don't think this is one absolutely necessary. It's nice and helpful, and perhaps some artists need it more than others, especially when they don't like the idea of doing the entire workload themself, but I worked with a writer on another comic for years and he didn't make much art (maybe a couple of little pieces of art for reference) and couldn't really help with the process.
However, on the other hand he had a wife and sometimes other artists who occasionally made concept art for me to go by, though even when that wasn't available, it helped that he was very informative and gave me what references he could.
My suggestion after collaborating with a writer for some time? Have the ending of your story in reach. It just makes it more likely that the artist(s) will be able to finish it for you without life happening or....in my case just hitting a wall with the entire project. ^____^;;
Also remember to give your artists some creative breathing room, especially if they're doing the entirety of the pages themselves. My writer was very kind about letting me do my own thing with the layouts n' such. He still was honest and upfront when something wasn't working for him, but he didn't jump all over me when I didn't make something exactly as he originally pictured it either.
Yeah, I'll stop editing this now. XP