When should you be using Web Workers?

The main thread has other responsibilties in addition to running a web app’s JavaScript. I said before that the main thread has other responsibilities in addition to running a web app’s JavaScript, and that’s the reason why we need to avoid long, blocking JavaScript on the main thread at all costs. This way they are enforcing both patterns: You always have to chunk your work into tasks so that it can be put in a queue, allowing the UI thread to attend to its other responsibilities whenever necessary, but also allowing you to run non-UI work on a different thread simply by putting the task into a different queue.

Source: dassur.ma