Steve,
Because a sacrificial anode is the easiest way to prevent damage to the important parts.
The anode is the part of the battery which loses material during electrical activity.
By attaching a piece of a "baser" metal, it corrodes instead of the important parts.

That mast looks like classic galvanic corrosion to me.
The base metal almost always corrodes from the inside and then the cracks become visible from the outside.
I have seen crossbars break at the location of pop rivets.