On the 15-22, the endcap MAY come in contact with the barrel when a bipod is added. It also may not allow the barrel to FLEX as much when actually shooting, which will alter the barrel whip, and absolutely can contribute to accuracy via a point of impact shift.

However, since one often isn't shooting 'match' 22lr ammo, and the firearm is 2-4 MOA anyways, it's really hard to say that the endcap would have (or not have) a significant impact on accuracy. Some people just 'believe' that it causes issues. The bottom line is that if it impacts your accuracy (or perception of it), then feel free to remove it.

In Wood714's picture above, the endcap is not just decorative; on a direct impingement firearm such as the AR-15, the gas tube and block need to be aligned correctly. The gas tube fits into the top hole in the endcap, and assists in keeping it aligned into the upper receiver. Since the 15-22 is a blow-back action, the endcap serves only decorative purposes (although I guess if you are dragging it through the mud, it might let a little less mud inside the handguard).

Remember to CLAMP THE BARREL when removing the flash hider (necessary to remove the endcap). Failure to do so on the 15-22 can result in upper receiver damage.