A controlled vocabulary provides a consistent way to describe data – location, time, place name, subject.
Controlled vocabularies significantly improve data discovery. It makes data more shareable with researchers in the same discipline because everyone is ‘talking the same language’ when searching for specific data eg plants, animals, medical conditions, places etc
Vocabularies are so important for keeping descriptions consistent. Metadata is actually of little use in terms of structuring and organising information if the language isn’t consistent across different records. There’s also a really important point on the JISC site – vocabularies seriously improve information retrieval, but only if the terms are obvious and meaningful to your users. Subject terms need to match the kind of terms that users will search for. Or, if users are referring to a subject thesaurus to search, they need to know that the terms they chose are the most relevant.
I had a look at this record in Atlas of Living Australia, after choosing my favourite magpie: http://biocache.ala.org.au/occurrences/8425ef9b-8aef-4798-8218-d96a04716c24
Pretty comprehensive metadata, and quite obviously sourced from scientific thesauri.