General William Tecumseh Sherman's 1864 march to the sea was more than just an empty act of brutality. It had a clear military purpose: depriving the Confederacy of the ability to make war.
The American Civil War being the first real industrial war, railroads were a key component of military logistics. Farther north in Virginia, Robert E. Lee's Army of Northern Virginia was supplied from further south by rail. He received enough supplies to hold back (or at least slow down) Ulysses S. Grant's campaign towards Richmond, the Confederate capital.
The destruction of Georgia's railroad infrastructure, then, became part of the tactics of total war. As Sherman's soldiers came upon railroads during their march, among the tasks assigned were blowing up locomotives, burning boxcars, wrecking machine shops, and tearing up the rails.
Now, if the rails had simply been left lying around, someone could have simply come along and rebuilt the railroad using the same rails that had just been torn up. And so, the rails were treated to further effort. Once a rail was torn up, soldiers (or recently-freed 'contraband' slaves) would place it over a fire of railroad ties. Once the rail became soft in the middle, it would be removed from the fire and then twisted around a tree or a wooden post. A 'Sherman's Bowtie' was the twisted, useless product.
To the rebels, of course, this bit of military expediency was viewed in a manner similar to the Romans' sowing salt on Carthage after that city's destruction. But it certainly made moving materiel difficult, and the destruction of railroads contributed towards the eventual capitulation of Richmond.