T-Mobile is a strong believer of its recently acquired 600MHz spectrum being the great equalizer to closing the gap between its network and those of its close rivals, AT&T and Verizon. Both AT&T and Verizon have the best networks in the US - depending on the metric, one is above the other but not in every metric - and that is largely due to its low-band spectrum that both carriers have. Between the two, nearly three-quarters of the US' low-band spectrum belonged to those two companies, before the Incentive auction which ended earlier this year.
A few years ago, T-Mobile picked up some 700MHz spectrum from Verizon and had been grabbing more from other regional carriers to extend that 700MHz footprint. Prior to this, T-Mobile did not have any low-band spectrum (this was mostly due to its parent company, Deutsche Telekom not taking part in those auctions). Since T-Mobile started building out its 700MHz network, its network has gotten much stronger, and surpassed Sprint, and many reports show that it is faster than Verizon and AT&T now. The only real issue left for T-Mobile, keeping it from being the best is its coverage. Verizon and AT&T cover more of the US than T-Mobile or Sprint. And that's where 600MHz comes into play.
During the Incentive Auction, T-Mobile picked up a ton of 600MHz spectrum, it picked up more than everyone else combined. And it is already building out this spectrum, as you'd expect. The 600MHz spectrum is going to help T-Mobile improve its coverage, especially indoors. As low band spectrum is better at offering more coverage with less towers, but the bandwidth is smaller. Meanwhile high-band doesn't cover much, but has a ton of bandwidth. Which is why it's important for 5G. An analyst, Nils Paellmann stated at a conference in Boston this week that with 5G coming, a lot of the LTE radios made for 600MHz will work with 5G through a simple software update on the device. Meaning that these 600MHz compatible devices coming later this year and next year will be perfect for 5G in the next few years. Which is the next step for T-Mobile and it could help it accelerate its 5G efforts.