Opens up more pipes in the bandwidth line and allows for faster speeds. Right now, Verizon uses Band 13 I believe, but now that 99% of the country is on that band with Verizon, they are ready to open up some new lanes on the highway, which is what Band 4 is. Phones older than the S4 won't have it with a few exceptions here and there.
This also allows you to use T-Mobile's frequency as well if you want to switch SIM's with T-Mobile since they're system is already in Band 4. Some of the other companies also use Band 4 in other countries, which makes for having more LTE options available. The Maxx is a world phone at the same time.
You should see significant speed increases. Right now though, New York is the only city that has Band 4 Verizon I believe, with the other major cities coming this November.