One of the central advantages to a monolingual dictionary is the depth of meaning inherent in the definitions. Monolingual dictionaries have the freedom to expand on initial definitions using multiple sentences. In addition, they can provide multiple definitions for a word based on the potential context of its meaning. Some monolingual dictionaries even include example sentences for each word defined.
Language learners typically use multilingual dictionaries to use to rapidly look up and translate words. However, these definitions can serve the learner best in the initial stages of her education. Once the learner has reached a more advanced level, a native dictionary is a better way to comprehend the true meaning of a word. Multilingual dictionaries, on the other hand, usually provide a more cursory definition that directly translates a word in one language to another word.
Another advantage of a monolingual dictionary is a potential expanded vocabulary of words. While multilingual dictionaries are focused on utility and communication, monolingual dictionaries have the advantage of being able to focus on more complex words, such as scientific ideas or rarely used terms. Monolingual dictionaries may also have the luxury of an expanded list of medical terms, while a multilingual dictionary would only have the most commonly used words.
Monolingual dictionaries are also more likely to have recent vocabulary. In an era when technology is changing our vocabulary faster than ever, this is especially important. Words such as blog, tweet and more recent slang are much more likely to be in a monolingual dictionary to reflect its users' deeper knowledge of the language.