MongoDB is a NoSQL database driver became popular while people were looking alternatives of MySQL and there were just a few alternatives to go for.
Since it has been released in 2009, it's still used with many different programming language for different functionalities combined with other databases to reduce their loads on transactions doesn't have "importance" usually.
Also, they have advertised it to Developer Communities with some famous projects like Foursquare to grow their audience and make technology used by a wider community.
MongoDB is one of the NoSQL database used by quite amount of developers for Document Storage.
The database structure of MongoDB has tables similar to relational databases, it's being called as "Collections" and inside these collections, data is stored in rows without having any columns.