In real life Sharepoint applications it is common practice to have consistent global navigation (or top navigation bar) across site. It is achievable out of the box with Sharepoint navigation architecture. But it is also common situation when site collection (SPSite) contains multiple sites (SPWeb) and there is a requirement to have consistent navigation when users goes through sites within site collection (it can be navigation items from one of the particular sites, e.g. site collection root site, or it can be navigation items retrieved from custom storage like xml file, database, etc). More complicated case – is when navigation should be preserved within multiple site collections.