[iam]

Yes, this is so blindingly obvious that I'm surprised we don't see it happen more often.

It doesn't block the UI and yet it still gives the user an indication when the actions are in progress/completed.

[splicer]

Example: the progress bars Gmail uses when you add several attachments in quick succession.

[iamwil]

Technically, seeing the spinner doesn't block you from doing other things, if there are other UI elements that exist on the page. Problem is, sometimes, web devs make the entire page a lightbox, and you can do nothing else except watch the spinner.