You don't need it to be very fast just to respond dynamically to some event handlers and whatnot. (The basic scripting stuff JS was intended for.)

You also don't need modern wheels, when just pulling a simple carriage wheels were invented for.

Modern JS engines are not written for playing funny tricks with "window.status" or "window.title" as in 1990, but for more complex applications.

Not all uses are useful, but that's to subjective. As with wheels.