why should use MongoDB
Mongo is NOT an RDBMS. Don't treat it like one and you'll avoid a lot of mistakes. This is why applications that require an RDBMS don't support Mongo.

MongoDB is well suited to applications where:


The working set of documents (and their indexes) can be kept in RAM.
Document sizes do not change much between creation and steady state.
The schema needs to be flexible.


It can definitely be used in large applications, with the right engineering. It depends a lot on how the application plans to use Mongo, though. With sharding to spread write loads you can get to some very high throughputs.