Validation, mock server availablity, ease of access... Multiple policies for confining the rights on your API, multiple version availablity.

I like RAML because, it gives a flexibility to the developer to customise micro services into convenient way. Designing specification follows a simple YAML structure(well known industry approach) which is easy for anybody to understand as it just contains nouns(i.e. resources) and verbs(i.e methods). RAML is strongly capable to support Security traits and schemes which can be appleid at different levels of API, example API level, resource level and method level. Modularisation: RAML is pretty much easy to define specification into individual modules such as fragments(nothing but a common module), traits, resources types, datatypes and as an API. This helps in de-coupling the specification to reduce unnecessary dependencies and keep the API clean for easy understanding.

Fast visualization of API design, You can easily create contract and send to your client for fast turn around. Good for AGILE methodology. For developers, I can say that it's readable and can be easily changed. It's maintainability is high and specifications of RAML is complete with a detailed guide and tutorials. It is precise, a client can be generated from the spec that's as faithful to the API.