z-index: 100 for overlay – why?

I recently realized that the default style for the overlay contains the following rule (along with others):

.embed-privacy-overlay {
  z-index: 100;
}

However – this causes the embed block to stick above pull down menus on my website. As a workaround I added the following rule in my theme CSS to fix that:

.embed-privacy-overlay {
  z-index: inherit !important;
}

But I wonder what was the original reason the z-index? So far I don’t see any issue with a default z-index. Embedding YouTube, Vimeo and Twitter work fine without any issues.