I'm not sure if it's important from an SEO standpoint, but I know that personally, as a browser, whenever I find a site I like and want to read more of, the first thing I look for is their site map/index/archive, so that I can find more articles I want to read.
If their site is very big and it's difficult to link to all pages from one page (for example it's difficult to list more than 500 links on a page), than at least they should have a "Top 100 Best/Popular Pages" list. Or even a set of pages, each one an index for each category, and one page to list all the index pages. I don't care how they go about it, so long as they have made some way for me to locate their pages.
And if they don't have some sort of map or index or archive, or if I can't find it easily, I'm prone to leave their site without bookmarking it.
I'm an author and I often am researching for my novels. If I'm writing a murder I need to know the details of how specific weapons/poisons/etc work. If I'm writing about Scotland or Persian or someplace else, I need to know the culture, folklore, cuisine, traditions, etc. If it's a Historical time period I need to know that info as well.
Well, because I'm on a tight publishing schedule I'm limited to how much time I can spend looking for certain pages on a site. I expect the webmaster to have laid out their site in such a way that I could browse it quickly to find what I'm looking for.
Then I'll need to go back to their site many times over the course of the writing of the novel (4 months or more) and thus I bookmark the sites that are useful. But if the site annoys me by not being easy to find pages important to my research, I'll get frustrated and not go back to the site and look for info elsewhere.
Thus, I prefer sites that make it easy for me to find their pages, thus, as a web browser, I prefer sites that have some sort of method for making it easy to find their pages. Thus I prefer sites to have a site map index.