This is very important because if you create blog.hostgator.com and blog.seohosting.com on the same cPanel, leave the Document Root as the cPanel suggested location, both of your blog subdomains will load from the same directory of /public_html/blog. You will not have separate content for each subdomain.
Furthermore, if you own several domains and wish to have a different blog set up for each domain that loads from the subdomain name blog, you will have to change each domain’s document root.