Android phones, apps and tablets

http://www.appbrain.com/app/com.rovio.angrybirds_lite?install

But for this as an example, it has the install link or barcode image to install.
Why would they supply that if it's not available on Market?

If its apps you're talking about, not all are on the market, and if you want them,
you'd have to go to the maker's website (in some cases) to get it from them directly.