What are the practical uses of ontologies?
I don't know about robotics, but ontologies are part of the standard toolkit for modern expert systems, especially those with a natural language processing component.

For example, consider the process of performing literature searches for systematic reviews in medicine. Of the millions of medical studies out there, reviewers need to find the 20 or so high-quality randomised control trial studies which are relevant to the clinical question that they want to answer. This is a classic needle-in-a-haystack problem, and computer scientists would like to build tools to aid the search.

Medical ontologies such as MeSH, ICD, and GALEN play a role in this, because they represent an "answer" to the question of what a given medical study is "about".