Spectrum is one of the biggest considerations for wireless carriers right now, and it would seem that AT&T has more of it floating around unused than anybody else; about 65MHz, to be specific, according to Deutsche Bank. According to AT&T, this number was around 40MHz as of last year, so Deutsche Bank's figures may or may not be entirely on the money. In either case, that's still a good bit of loose spectrum. AT&T's network is quite robust without that spectrum, according to a recent report from OpenSignal, which means that they can put that unused spectrum to work mostly as they wish. Naturally, this gives them in advantage in the 5G race, and leaves them with a huge pool of available spectrum to use for patching holes in their network or adding coverage where there once was none.
On top of the sheer amount of normal spectrum that AT&T isn't using, it will be freeing up more in the near future by sunsetting its 2G networks, and there is a deal with FirstNet reportedly in the works that will free up much of the spectrum that AT&T currently uses for emergency services. Deploying fiber in more areas can also help them to take some of their fixed wireless customers off the books, freeing up more spectrum. Ongoing experiments with millimeter wave spectrum, meanwhile, will open up a whole new world of spectrum use to them, and help them to get more out of the spectrum holdings that they already have.
We are fast approaching the age of 5G, which means that carriers will have to refarm tons of spectrum in order to build out networks adequately to meet consumer demand. For those unfamiliar with the process, it involves a great deal of paperwork and permits, along with the required work on the physical and software ends to rip the spectrum in question away from its current task and, in essence, tell it to do something else entirely. While many carriers also have a dearth of loose spectrum and some are already building out networks based on current 5G standard drafts, AT&T's massive quantity beats the competition, giving them an advantage when the starting gun finally fires for in-earnest 5G buildouts.