There are a variety of reasons why you’d want to create multiple site collections – to avoid recommended capacity limits, to provide a logical site structure, etc.  One of the drawbacks with creating multiple site collections is the lack of out-of-the-box functionality to access and share content across site collections.  While portal site connections don’t do anything to access or pull content, it does allow you to specify a “connection” to another site collection, which appears in the global breadcrumb.  This makes logical navigation easier.  Consider the following example:  a company needs to have separate site collections for each department (HR, accounting, IT, etc.), but there is also a “top-level”, shared area of the environment, which will be a separate site collection.  When you’re in each of the departmental site collections, it would make sense to have a link back to the top-level site collection, to insinuate a logical hierarchy. Simply put, a portal site connection gives you a breadcrumb link to another site collection.

Let’s take a look at the above example.  I created a root site collection (http://intranet.devexpertise.com), and also an accounting site collection (http://intranet.devexpertise.com/sites/accounting). When you’re on the accounting site collection, there’s no visual indication or link back to the “root” site collection:

By simply adding a portal site connection, we can provide a visual indication of the logical hierarchy and link back to the root site collection.  To add a portal site connection, navigate to Site Settings > Site Collection Administration > Portal site connection, and specify a URL and a friendly name for the link:

Now, the breadcrumb will show a link to the root site collection:

Great! …as long as you’re always going to access your sites from a single URL.  What if you have multiple URLs set up, such as the situation if you allow external access to your environment and have a separate external URL (http://extranet.devexpertise.com).  The logical thing would be to just specify a relative link for the portal web address, however SharePoint won’t let you, and pops up a nice “Please enter a URL for the portal site” error:

Why is this really a problem?  Well, let’s say you left it as http://intranet.devexpertise.com, and accessed it from http://extranet.devexpertise.com.  The portal site connection link would still point to the intranet!

Lovely, huh? Fortunately, you are able to easily set this to a relative link using a few lines of code:

using (SPSite siteCollection = new SPSite("http://intranet.devexpertise.com/sites/accounting"))
{
    siteCollection.PortalUrl = "/";
}

That’s it!  Now, no matter what URL you access the accounting site from, the portal site connection link will jump you back to the correct root site collection: