In case you haven't noticed, T-Mobile's spectrum is not the best. Typically the lower the spectrum, the better building penetration you'll get with that network. Which is why Verizon and AT&T's network have the best building penetration due to the 700 and 850MHz spectrum they own. Although Verizon owns a ton of spectrum. They also purchased TV spectrum not too long ago, and in doing so, they agreed to sell off a block of 700MHz spectrum they owned. We already know that T-Mobile might be interested in it, which makes sense, and would be a great thing for magenta. The spectrum is said to be worth about $4 billion. But Magenta isn't alone here.
There's a new report coming out that says AT&T might also be interested in that particular low-level block as well. AT&T already has a block of that level spectrum and probably has the means to absorb even more of it. This level of spectrum gives off a penetrating signal, while the higher frequencies have a stronger data transfer, but not necessarily the same coverage.
If T-Mobile were to acquire this spectrum, they'd immediately grow their coverage, especially in non-Metro areas. Which right now, is the biggest complaint I hear about from people that aren't on T-Mobile. T-Mobile currently uses the 1900MHz realm, which does give off a stronger signal, but then the coverage is pretty limited. This would also solve a nagging issue of T-Mobile operating outside the "normal" coverage frequency.
AT&T can buy this spectrum, but they don't need too. The only real reason I see for AT&T to enter the auction for this spectrum is to block T-Mobile from getting it. Which I wouldn't put it past them to do so. T-Mobile desperately needs this spectrum to grow their coverage, and thus bring in more customers. But when all is said and done, Verizon will be pretty happy with this bidding war.