A site administrator can access every sub-account for a specific customer site as well as create and manage administrators for the site. This includes managing what permissions and Roles account administrators should be assigned. Site administrators are the only administrators with access to edit or create Roles for the account. They are also the point of contact for the site, and accounts that belong to that site.
Site administrators have access to advanced reports in the Analytics dashboard like Planned Mailings, Insights, and Custom Reports (depending on license level) which allow them to view all of their scheduled and draft campaigns across all accounts on their site. They can create Report Groups that span the entire site to create cross-account reporting.
If you are a multi-account client and have a Customer Template Repository enabled, site administrators are the only users that can save templates from sub-accounts into the repository to share with all other sub-accounts.
And finally, Site administrators automatically have access to new product features when they are released.