Absolutely. But if a web author quite reasonably adds a symbol for the benefit of the sighted majority, using an unlabeled image, an icon font, or coopting an existing symbol (e.g. degrees), that leads to extra clutter or confusion for blind users. If the symbol is in Unicode, we can have a solution that works well for everyone.

This symbol is for marking external links, distinct from internal links.

Right. And I've come across websites where the alt text for their external-link image is something fairly verbose like "external link opens in new window". Now, imagine if instead, we had a standard symbol, and screen readers could map it to short sound effect that users would come to recognize.