Samsung, HTC, and LG all have apps that are dedicated to their particular Windows Phones. Manufacturers offer these apps as a selling point, hoping that such apps will attract consumers to thier Windows Phone. There are some really nice apps from each of these manufacturers but they are confined to that particular device. I can't load Samsung's Photogram on to my HTC HD7s or HTC apps to a Samsung.
In a recent interview, Nokia's Senior Vice President of Developer and Marketplace, Marco Argenti, said this won't be the case with Nokia Windows Phones. Quoting,
"Every Nokia App will be available on every phone."
He would later clarify his comments by saying that while the Nokia apps will work on any phone, there will be exclusive to Nokia Windows Phones for a set time. Argenti didn't get into specifics about what Nokia apps will be available but did comment that they are working hard to enhance areas such as mapping, commerce and discovery.
We'll have to wait and see what apps Nokia has up their sleeve but maybe having access to Nokia apps from other Windows Phones will be a catalyst. Maybe other manufacturers will follow suit and we'll see HTC, Samsung and LG apps popping up on the Marketplace.