Refer to MONGOID-5660 for context.
 
Hash#to_criteria is a kernel Monkey patch that converts a Hash to a Criteria by currying its key-values to an initialized Criteria class. It looks like it was done in order to support passing in a hash as an arg to Criteria.merge!, there's not a major use for it otherwise (Criteria also doesn't define #to_h)
 
PR raised: https://github.com/mongodb/mongoid/pull/5717