It looks like it is just an interface standard - a set of interfaces that must be implemented by an ECM system in order to be considered compliant, and that make it easier for data to be shared between systems.
Say, for example, BigFish, Inc. buys out Guppy, Inc. BigFish has a document repository based in Microsoft Sharepoint, and Guppy uses some other system. BigFish is going to have to pay some software consultants good money to develop a custom solution to integrate the two pieces of software, and allow BigFish to access Guppy’s document repository through their Sharepoint software. The software consultants will have to dig deep into both applications to figure out how they work, and how to get data to transfer between them.
However, if both applications are CMIS-compliant, that means that they are guaranteed to offer up specific services in a CMIS-standard manner. Integrators won’t have to dig deep into application-specific functionality to figure out how to extract data from one of the applications - they’ll know that the application supports the CMIS standard web service for retrieving that data. Integrating the two applications will be much easier, because they both support the same standard interfaces.
You could almost think of it as if each ECM application spoke in it’s own language: English, Spanish, Chinese, etc. CMIS would be like Esperanto, and if all the vendors agreed to learn it in addition to their native language, then all of the compliant applications would know how to talk to each other.