1:1 Pixel Matching is required to ensure that a video signal that is identical to a display's native resolution is delivered to the display on a matching, pixel-by-pixel basis, without extraneous processing or scaling that could introduce visible artifacts.
For digital displays, 1:1 pixel mapping is critical to obtaining artifact-free images. To illustrate why 1:1 pixel mapping is an important element in picture quality, let's look at a common scenario.
Let's say that you recently purchased a DLP television with a native resolution of 720p and that you have an over-the-air digital receiver that is capable of outputting HD programming in the native resolution of the broadcast. As you may know, most ABC and FOX local digital stations deliver their HD broadcasts in the 720p format, while CBS and NBC local stations typically deliver a 1080i HD signal.
With an over-the-air receiver that outputs in native format, this means that your receiver would deliver a 720p signal to your television, without any conversion, as a 720p signal; the same would be true for a 1080i station -- the receiver would pass the 1080i signal to the TV without any conversion.
In this example, the TV will still have to scale the 1080i signal to the display's native resolution of 720p. But when it comes to the 720p signal, the TV -- if it is capable of 1:1 pixel mapping -- will "pass through" the 720p signals without any further scaling or processing.
Avoiding extraneous processing through 1:1 pixel mapping is important for two reasons:
First, anytime a signal undergoes processing, there is going to be some level of degradation; usually that degradation is minor, but the effect is cumulative and therefore it should be avoided whenever possible.
Second, the built-in scaling circuitry included in most consumer displays delivers relatively pedestrian scaling performance, so there's no point in subjecting a native resolution signal to processing that will degrade the integrity of the original signal, even if that loss is minor.