This is conceptually similar to any.tags(), any.notes(), and any.description(), but allows the storage of an object rather than a string. This is valuable because, besides annotating the schemas, it allows us to build modules on Joi that provide additional functionality. Example use case:

var schema = {
  _id: Joi.number().integer().meta({index: {unique: true, sparse: true}}),
  name: 'Margaret'
};

// Registers schema as the validator for person models, and
// extracts indexing information for each field from the metadata on each field.
myDBAdapter.model('Person', schema);

This can currently be accomplished using tags(), but it requires string parsing and complicates the logic of actually retrieving the metadata. E.g.

var schema = {
  _id: Joi.number().integer().tags(['index:unique', 'index:sparse']),
  name: 'Margaret'
};

The benefit of an any.meta() is similar to that of the existing description functions -- the metadata is stored consistently and doesn't need to worry about clobbering / being clobbered by Joi's moving parts.

Cheers,
Josh