See the photo bellow I took off Yachtworld.
Of the modern boats of today, the Jeanneau Sun Odissey 37 for example, is one boat that has it. The Sun Fast, which is basically the same boat with taller rig and diffetent keel and more oriented to racing does not have it.
It is stiffening the lower part of the mast, by preventing it from flexing backwards. It seen often is cruising boats and normally racing boats don't have them, or if they do, they're "flying stays", meaning they can be de-tensioned.