It helps you get a sense of your application's schema, as well as any outliers to that schema. Particularly useful when you inherit a codebase with data dump and want to quickly learn how the data's structured. Also useful for finding rare keys.
### An Easy Example ###
We'll make a collection:
db.users.insert({name: "Tom", bio: "A nice guy.", pets: ["monkey", "fish"], someWeirdLegacyKey: "I like Ike!"});
db.users.insert({name: "Dick", bio: "I swordfight."});
_("test" is the database containing the collection we are analyzing.)_
Hmm. Looks like everybody has a "name" and "_id". Most, but not all have a "bio".
Interestingly, it looks like "pets" can be either an array or a string. Will this cause any problems in the application, I wonder?
Seems like the first document created has a weird legacy key- those damn fools who built the protoype didn't clean up after themselves. If there were a thousand such early documents, I might cross-reference the codebase to confirm they are no longer used, and then delete them all. That way they'll not confuse any future developers.
Results are stored for future use in a schemaAnalyzerResults database.