ICBMs are a large, blunt weapon. Their value is primarily in their deterrence. The concequences of actually using them are so unthinkable, that we don't really consider it unless it's a very-last-resort kind of situation. Most of the time you want a precision, guided munition that can destroy a house or a truck or something small without causing too much collateral damage. Until now, that has meant an airbase or a carrier nearby to launch aircraft to carry these munitions.

However, with the XR-72, we can do away with those and just launch our precision strikes from the US mainland. It's cheaper, just as effective and hopefully will give emerging superpowers (like China) pause when planning their defence scenarios.