Once in a while, I encounter some people (even native speakers) quote from informal dictionaries like this one: http://www.urbandictionary.com/
I used to ignore this type of dictionary because I suppose the contributors are normally ordinary or average people. Although many words and definitions may have also been contributed by a few language experts, you cannot distinguish which ones are because anyone can make contributions.
I prefer to consult and quote from reputable dictionaries like Oxford, Merriam-Webster, American Heritage, etc. After all, they hire the best in the field of linguistics and lexicography, so I feel that their definitions are far more reliable.
But then I realize that informal dictionaries are also useful, especially for slang and very informal terms, if I want to effectively communicate with people of all class. When you look up dictionaries like this, (usual) ordinary people will tell you what they think or what they mean when they use particularly new words, since language is constantly evolving.
The problem with regular and academic dictionaries sometimes is that it takes time before they add new words and expressions to the dictionary. For example, M-W editors need to see a substantial number of citations in printed materials usually over a long period of time before a word is considered. https://www.merriam-webster.com/help/faq-words-into-dictionary
I believe this is true in almost any regular dictionary. I don't think they will immediately add a new word with few citations, but there may be an exception to that.
Here is another problem: ESL teachers and students will usually consider a term or expression (being used by many native speakers) incorrect or nonexistent if it does not appear in their dictionary.
If you're an ESL student and would like to effectively communicate with ordinary native speakers, an informal dictionary can help you determine what they mean when they use a particular term or expression that does not (yet) appear in a dictionary. (Maybe that word or expression is still being considered by editors or it did not pass the criteria at all, so it won't be added to the dictionary anytime in the future.)
I would love to hear your thoughts about this topic.