Why does radix tooltip duplicate the tooltip content?

[wbolduc]

The above screenshot is taken from the official tooltip sample
https://www.radix-ui.com/primitives/docs/components/tooltip

Why does "Add to library" appear twice in the tooltip?

[wbolduc]

Popover doesn't seem to duplicate the content.

I'm assuming the duplicated content for tooltip is some accessibility thing I've never considered... but what does it do?