It's a magical bullet point on a list of features that proves that your game is next-generation, and there are groups of players online who sneer at anything that can't quite hit that goal.
Diablo 3 on the Xbox One, for instance, couldn't quite get there. The original response was to drop the resolution down to 900p and keep the frame rate. A smooth game, many would argue, is worth sacrificing a few lines of resolution. The problem is that this solution created a PR problem for Microsoft, a company that is scared of consumers seeing the Xbox One as a less-powerful console compared to the PlayStation 4.