It runs in a sandbox. Evolution has an HTML editor. You can embed images and links into the document that you're sending. However, when you receive such a document, you will only get images which are part of the message document. Evolution makes less use of it.
Evolution uses the pair in conjunction to find components available on the system.
If there are no components which can display something in Evolution, it can look for programs to perform the same function. This is nice because you can achieve consistency between Evolution and Nautilus, apparently. I imagine they were thinking of things which you are happy to run on a local machine but which you wouldn't run across the network or something. Evolution has a split between 'model' and 'view'. The model contains data.