Their purpose is to evaluate an interface to check whether it follows some common principles to make it accessible to users with disabilities. Besides those principles, the experts may use other tools that run automatically or semi-automatically.
These experts have deep knowledge of different accessibility standards, and which one to use for every situation (including some legal considerations, like accessibility levels that must be achieved by law).
Context of use:
Accessibility experts use the principles and guidelines as a support when evaluating interfaces, but they will probably use other tools which make some checks automatically.
Some kind of integration with external tools for evaluation would be a useful point.