I've read about OWL - Web Ontology Language (few pages only), but what are some real-world use cases/implementations for it?
There are a large number of use cases. The web ontology language is a language for formalising ontologies, with a particular view for use on the web. As such there are two ways of interpreting your question: what are the use cases for ontologies and what is the use case for OWL (the language specifically). Tackling each in turn:
Ontologies are computable specifications of a domain or a conceptualisation: they are computable models of the world. As such, they are used for, for example, data integration (if data is labeled with an ontological type as metadata and there are enough specifying axioms, a machine can compute whether, for example, two pieces of data refer to the same thing/concept and should be combined etc...) or faceted searching and Description Logic based querying.
The use case for OWL the language is, that it allows ontologies to live on the web (OWL is part of the semantic web tool stack) by using URIs for classes, axioms etc....there are many other ontology languages such as OBO, for example, which do not allow you to do this. If you are wondering why that is a good thing, think about open linked data etc.