Because the television manufacturers say so, might be the thought on the minds of many. It's true that TV manufacturers seem to latch on to something new every year and then cram it down the throats of consumers, see 3DTV.
Do we need it? Sure, the content will look all that much better, for the consumer that has a 4K screen and a setup that will allow the 4K content to get to the screen. How many of you have a 4K screen currently? Not many I'll wager. Television prices are ridiculously high still, most people won't be buying them this year, maybe not even next year. But if you've been lucky enough to attend a CES in the past couple years, you've seen a 4K TV in action. It's extremely impressive. They seem extremely bright and the images are indeed quite a deal better than 720p and 1080p for that matter.