The sub-domain of your site is directly affected by PageRank. If your subdomain is high, it is also good for PageRank. Your subdomains, along with your main domain name, must be high quality. You can go up in search engines or improve your site.
The sub domain has been shown to increase the quality of the main domain. It has even been seen to be more influential than the main domain. In such a case the subdomain contributes to the main domain.