A PromiseWorker is a ChromeWorker except instead of calling postMessage() to send a message, you call post(), which returns a Promise. JavaScript files imported into the worker scope and main thread scope which allows posting to the worker and receiving in the form of a promise. Like ChromeWorker objects, PromiseWorker is mostly used for js-ctypes but it is not limited to that. There is no XPCOM access, similar to ChromeWorker. This answers the question "When should I use a PromiseWorker?", and the answer is, whenever you would normally use a ChromeWorker, but want postMessage to return promises.
As web applications become more and more powerful, adding features such as audio and video manipulation, access to raw data using WebSockets, and so forth, it has become clear that there are times when it would be helpful for JavaScript code to be able to quickly and easily manipulate raw binary data. In the past, this had to be simulated by treating the raw data as a string and using the charCodeAt() method to read the bytes from the data buffer.