When you create or refresh a developer sandbox in Salesforce, records on standard objects such as Webmerge Mapping aren't included in the new sandbox. These mappings are often a critical part of processes that need to be tested end-to-end. In these cases, mappings need to be recreated manually. This can be a painstaking, error-prone process when there are complex mappings involved. Any updates developers make to mappings can't be deployed via typical Salesforce deployment tools.
Since the Winter '16 release, Salesforce has offered Custom Metadata Types. These are similar to Custom Objects, but they have the key advantage of being deployable. Custom Metadata records that exist in production are replicated in sandboxes when they are created or refreshed.
Having the option to define mappings with Custom Metadata Type records instead of Custom Object records would be a huge advantage for teams that need to incorporate Webmerge into the full Salesforce development lifecycle.