General encyclopedias cover a range of topics across subject areas. These can be a good place to start if you not sure what subject area covers the person, event, country, concept, etc. you need to look up.
Reading a literary work set in an unfamiliar time period or location? Reference works can help provide a brief introduction to countries, their history, and even to specific historical events. Knowing its context can bring an added layer of meaning to a literary work.
The English language has changed over time. Words we use today may have meant something quite different in Medieval or Early Modern times, or may not have been introduced into the English language until much later. Some words that were used frequently in the past are no longer used today. Specialized dictionaries can help you navigate the range of words and their changing meanings over time.