255.4 is commonly referred to as METRO ('MEE-tro") and, as others have noted, is used mainly to relay weather information.
I'm splitting hairs here, but 255.4 is not METRO. It is indeed a FSS nationwide freq. 255.4 and the associated VHF freqs are all run by Lockheed Martin nowadays. If you were going through a Green Bay FSS node, the aircraft will call GREEN BAY RADIO as an example. Can do more than just get weather, but like you stated, that's what it is used for a lot of the time.
The METRO frequencies are tied to the weather shops at particular military bases and just about all on UHF. If the aircraft wanted to talk to the weather briefer at Tinker AFB, they'd call TINKER METRO on whatever the UHF freq is there.