main purpose, get rid of annoying errors caused by side effect imports

Analytics modules that run in the background, monitor user interaction, and send the data to a server.

Many jQuery plugins attach themselves to the global jQuery object.

A polyfill for example, might not do anything, because it finds that the feature that it enables is already supported by the browser.

A module with side-effects is one that changes the scope in other ways then returning something, and it's effects are not always predictable, and can be affected by outside forces (non pure function).

Import a module for its side effects only Import an entire module for side effects only, without importing anything. This runs the module's global code, but doesn't actually import any values.