Not that one wants to use nukes - but just having the option for certain situations is a strategic advantage.

For example...wiping out underground terrorist camps in the remote Afghan mountains...or taking out a whole nuclear facility/region in a rogue nation...or taking out the North Korean army if it makes a move on South Korea.

All these scenarios give the low-yield, tactical nuke a viable justification for existence.

Al Qaeda and other terrorist organizations want to nuke the western world, this gives us a very aggressive tool to use against terrorists without risking US lives or the chance of missing the target.

In a different world the US would not need to build such a weapon, but the world we find ourselves in is not that world. Had the cold war and the nuclear 'megaton race' with the USSR never happened - our arsenals would likely be filled with a variety of such low yield weaponry.