It's often necessary to run maintenance scripts on your database. Fields get renamed; data structures get changed; features that you used to support get removed; services get migrated. The list of reasons why you might want to change your schema is pretty limitless. So, the 'why' is pretty self explanatory.
The 'how' is a little more unfamiliar. For those people accustomed to SQL functions, the above database scripts will look strange. But notice how they're all in javascript, and how they're using the same API as we use throughout Meteor, on both the server and client. We have a consistent API through our database, server, and client.
Run the schema migration commands from the meteor mongo shell:
# run meteor
meteor
# access the database shell in a second terminal window
meteor mongo