You could, in theory, have a context that is entirely within the sandbox and useful. Hence my comment about getting things in and out of the environment potentially negating many of the scenarios I can think of. But broadly speaking, if you had some local processing to do, you feed the environment a blob and the environment can now pretend it's a normal file as far as it is concerned, and then you can pull the blob out when done. WASM can't touch real stuff but you can feed it stuff within the reach of javascript which itself is still sandboxed, but specific network touch points and user indicated file touch points can be put in the reach of javascript.
So if you wanted to apply, in browser, some linux utility to a file, then the user has to indicate a file for operating on via browser, and that action allows javascript code to access that file, and with that granted it can load it into some memory that you've allocated for this purpose, and when done move the data back or wherever.
But the much needed sandbox does greatly complicate things and for some sorts of files the resource usage would be prohibitive in this scenario.