Congratulations! A visitor has found your web site. They came across it while doing a Google search that landed them at a weird address (something like "http://blog.bigcorp.com/archives/2011/blah-blah-blah.aspx"). After spending all of 5 seconds on that page, they now want to see what your company is about.
So, they click on the big logo in the upper-left corner of the page but it just takes them to "http://blog.bigcorp.com/". That's not where they expected to go, so they leave, thinking that you didn't go all out and buy a "www" address to go with your "blog".
This scenario is how a typical person behaves on a site that cares so little about site navigation that they can't bother to put a link back to their primary home page on each and every page of their site.
Your average visitor isn't going to think to fiddle with the address bar and delete the "blog." and the "archives/...." bits. They just won't. If they type something up there, it's more likely going to be a different address, or a new Google search.
Today's example of this terrible oversight that is so oft-repeated around the entire web comes from microsoft.com. They're all excited about Windows 8 and how it's coming along and have set up a bunch of pages in the MSDN corner of their web site, found here:
It's a nice site. Lots of videos and information. Even a free beta copy of Windows 8 to download and play with. I'll wait here while you try to find a link back to "www.microsoft.com" on that page. They even have a Microsoft logo in the footer area of the entire subdomain. But they didn't bother to link it to "microsoft.com". Tsk, tsk.
Their "buildwindows.com" promotional site is another example. That home page also has no link back to "microsoft.com"; again, not even the "Microsoft" in the copyright shown in the footer (they also have many company logos on that page, also unlinked which is sort of a dis, frankly).
As I said, this happens all over the web. Apple is just as bad: their developer.apple.com site has no link back to "apple.com". Oh, they have a link in the footer to their online store there, but not to the home page of their site. For shame.
If you manage a web site that suffers from this problem, please fix that. It's easy to do and it makes for better navigation across your web site. And hey — it could improve page views and time-on-site (if you're into that)!
So, a rule of thumb: if you start at the home page of your site and click around long enough, you should never reach a spot where you can't return to the home page without resorting to the "Back" button.
Simpler rule of thumb: always include a link to your primary home page — preferably an obvious one — on every page of your site.