This might depend on the area. At least in Switzerland the answer is simple: You can use them for planned overnight stays.
They are usually just a small part of the entire hut with none to rudimentary to quite luxury cooking/heating facilities. Their size and equipment present varies greatly between different huts. Most SAC huts have one though. You should usually contact the "hut person" in advance, both for reserving (he can tell people if it is to be expected that too many mountaineers go there - this is usually not pleasant) and to receive sometimes vital information (e.g. where/which door is the entrance in case you have to shovel it free of snow).
Please follow the usual guidelines: Leave the place in equal or better state than you found it. Pay the fees and usually make an entry in the hut book - maintaining a hut involves lots of work and money.
According to Wills and Tullochgorum (thanks) this works more or less the same way in Austria, Italy and France.