Look at the one time nukes were used in war (so far) for an example. The US didn't need nukes to destroy those cities. Hiroshima and Nagasaki probably weren't even the deadliest bombing raids of the war: that honor goes to the attack on Tokyo on March 9-10 1945.
The difference is that it took 339 B-29s to devastate Tokyo, but only one B-29 to devastate Hiroshima and Nagasaki.
If you have a few hundred bombers armed with conventional weapons, they can destroy a large target. Arm them with nuclear weapons and they can destroy hundreds of large targets.
Battlefield nukes are similar. An attacking enemy tank division can be stopped by a division of your own... or a single nuclear artillery shell.
This was really important during the cold war because the USSR had massive superiority in conventional arms. NATO troops were significantly outnumbered, and they weren't willing to spend what it would take to reach parity. Nukes were a cheap way to defend. Instead of stopping invading Soviet troops with lots of your own troops, stop them by blowing them up.
Then it just escalates. If the other guy can nuke your troops, you need to be able to nuke his or else you'll lose the war. You want to stop the other guy's nukes, so you nukes nukes that can destroy their air bases and missiles, not just tactical nukes. If the war continues then supplying your troops becomes important, and you need to destroy their factories and transportation infrastructure too.
The MOAB weighs 21k pounds and is equivalent to 11 tons of TNT.
The W54 battlefield nuke weighs 51 pounds and is equivalent to 10-20 tons of TNT