For example, a number of aviation museums (e.g. CATPM, HWA, MAAFM) incorporate original (i.e. World War II-era) buildings which had a specific purpose (e.g. barracks, canteen, motor pool, infirmary, etc.), but are no longer used for that purpose. Instead they are operated as exhibits (If it helps, this is similar to historic building museums.)
On the one hand, tagging an infirmary as a medical building would, presumably be confusing for anyone looking for medical care. However, on the other hand, certain buildings, such as a motor pool, are defined by their overall design, like having large vehicle bays, and without major structural modification would be difficult to consider any other type of feature than garages. Note that this question is specifically in regards to buildings that may attempt to replicate their original function and not those that are now used for a well-defined, non-original purpose. (e.g. a factory converted into apartments)
Tagging every building as “museum” is not really appropriate, since they are usually all part of a single larger organization that encompasses the entire area.
P.S. You may consider this a follow-on to a previous question of mine. I have a very particular focus. 
