The selling point of the U2713HM is of course its resolution, just like with the U2711. The IPS panel has a resolution of 2560x1440 pixels, which is a lot more than the usual Full HD 1920x1080 resolution. The 27 inches result in a pixel density of 109 ppi, compared to 82 ppi for Full HD on a 27-inch screen.
It's a noticeable difference in terms of sharpness, but it still doesn't compare to notebook screens. Even the much-maligned but common 1366x768 pixels on a 15.6-inch display create a comparable 100 ppi resolution. Full HD resolution at that diagonal results in 141 ppi. That type of resolution on a 13.3-inch display, like on the ASUS UX31A Ultrabook, translates to 166 ppi. A 10-inch tablet with 1920x1200 pixels, such as the Acer A700, has 226 ppi. To achieve that kind of sharpness on a larger monitor, you need to look at resolutions like the 2880x1440 pixels on Apple's 15.4 inch MacBook Pro Retina. On a 27-inch screen not even a 4K resolution of 3840x2160 pixels would suffice to achieve such a degree of sharpness.
What these extra pixels enable is an increase in sharpness, by utilising the extra pixels to display an apparently lower resolution with more details. You could compare this to how full-scene anti-aliasing works: by doubling (or tripling) the horizontal and vertical resolution and resizing the result back to the native resolution, extra ‘intermediate’ pixels are calculated, resulting in diagonal lines looking less jagged. In order to obtain the best results, ideally the resolution is an exact multiple of two in both directions, like the iPad 3’s 2048x1536 screen offering exactly twice as many pixels horizontally and vertically as the 1024x768 pixels of its predecessors. With less well-matching multiples however, the scaling effect can result in a seeming decrease in sharpness, as an unwelcome and unintended side-effect.
The added value of the extra pixels in monitors such as the Dell U2713HM is therefore not primarily the sharpness, but the extra desktop work space that is made available. With 2560x1440 pixels a screen like this has 3,686,400 pixels - considerably more than the 1920x1080 or 2,073,600 pixels of Full HD. About 77 percent more, actually. Of course, the smaller pixels result in everything looking sharper at the native resolution, but they also make everything on the screen look smaller, even if in the case of the U2713HM this is partially compensated for by the larger dimensions of the monitor.
This animated cut-out of part of the Hardware.Info logo shows the difference between standard resolution and twice as many pixels horizontally and vertically.