Web Workers allow for something similar to multithread operations that are common in languages such as Java or C++. In front end world, they are tool which is yet to be more frequently used.
That's, in my opinion, mainly due to lack of knowledge about their use cases, misjudgment that most people have fast PCs and modern browsers, and force of habit of sticking to what one knows (I second that). Additionally, there some limitations to their usage, such as the inability to pass functions into them.